『韓国語教育研究』（第 11 号）別刷

ISSN 2186-2044【研究論文】The First Language Acquisition processes ofCase Markers in Korean-Focusing on the results of an analysis of the CHILDES Corpus-RYU Ju-Yeon

日本韓国語教育学会
２０２１年９月

The First Language Acquisition processes ofCase Markers in Korean-Focusing on the results of an analysis of the CHILDES Corpus-RYU Ju-Yeon
This study investigates the early emergence and subsequent development of case markers
in Korean, ka/i (nominative), nun/un (topic), lul/ul (accusative), ui (possessive), and do(delimiting). It also examines the relation between children's age and MLU (Mean
Length of Utterances) in the early usage pattern of case markers. Longitudinal Koreandata, called the Ryu-Corpus, from the CHILDES database (MacWhinney, 2000) wereanalyzed. The data consists of longitudinal video-recorded interaction from three children(JONG 1;3-3;5, JOO 1;9-3;10, YUN 2;3-3;9) and their caregivers. Three children
produced 3,723 predicate tokens, which used the five case markers. The results indicatethat in the early stages of development, the three Korean children began to produce andproductively use from nun/un (topic) to do (delimiting) to ka/i (nominative) to lul/ul(accusative) and finally, to ui (possessive). All three children have shown the sameacquisition pattern of the case markers; however, there was a large difference among thethree children regarding the age of emergence of case markers. JONG started to use thetopic markers nun/un at 1;7 in his speech, whereas JOO and YUN started to use it at 2;10
and 2;7, respectively, taking more than one year after JONG started. As for MLU, there issome correlation between the MLU of the emergence of the case markers and their later
course of development. Three children started to use the case markers at 2.2~2.6 MLU,
and then used stably the case markers at 3.1~3.5 MLU after acquiring the possessive
marker ui. It suggests that the MLU can be the measure of the acquisition of case markers
in Korean, because there has been no study of MLU analysis on the L1 development of
Korean.

1. Introduction
In the first language acquisition process, as the development of grammar progresses,
it is said that this development is reflected in the length of sentences, withgrammatical structures becoming more complex and children starting to use various
conjugative endings and markers. Therefore, grammatical development can be
captured by mean length of utterances (MLU, the mean number of words used perutterance). Furthermore, while there are large individual differences in the speed of68language development, when children are compared using MLU, a similar
developmental order is observed. Brown (1973) clarified that when children learn
English as their mother tongue, there is a certain order, such as mastering the present
progressive “ing” or plural “s” faster than the third person singular “s” or passive
forms (Table 1). At that time of his writings, Brown used MLU to separate children's
language development into five stages, proposing MLU as an indicator ofgrammatical development. In addition to MLU, Communicative Development
Inventories (CDI) that measure the development of vocabulary until the age of 3–4
years and Lee’s (1974) Developmental Sentence Scoring (DSS) that measures
grammar are also used, but in this study, we will discuss the processes involved inlanguage development based on the more commonly used MLU.
Table 1. MLU Stages in the English Language (Brown, 1973)
Stage I(MLU1.0~)

Semantic roles (such as agents); no conjugation endings

StageII(MLU2.0~)

Modulation of meaning: acquisition of verb conjugation endings and pluralforms; acquisition of 14 grammatical morphemes

StageIII(MLU2.5~)

Single-sentence modulation: Acquisition of Auxiliary Verbs

StageIV(MLU3.0~)

Compound sentence structures: Complex sentence structures, such ascompound sentences

StageV(MLU4.0~)

In the case of Japanese, which has the highest level of similarity to Korean of any
language, what stages does the learning processes follow? Ogura (1999) and Miyata,
Otomo, and Shirai (2015) are cited as studies focusing on the acquisition of specificgrammatical items in the study of first language acquisition in Japanese. Ogura(1999) examined the number of people who used particles, auxiliary verbs, etc.,
more than five times in 19 min in a cross-sectional study of a total 60 people (groupsof 10 people every 3 months from ages 1;0 to 2;3). Table 2 summarizes the firstoccurrences of basic grammar items used more than five times by several people.
Since Ogura (1999) did not analyze the objective case marker "wo," the acquisitionof the objective case marker cannot be directly compared. In addition, Miyata,
Otomo, and Shirai (2015) used CHILDES's (Child Language Data Exchange
System) Japanese-language corpus (one-hour of natural speech data per week fromfive people) to investigate the acquisition order of particles and the conjunctive
auxiliaries of verbs, clarifying the relationship between this order and each MLU
stage. As a result, isolating and summarizing only those case markers that will be
subject to analysis in this study, it was found that they were acquired in the order of69

topic marker "ha" and modifier "no" then the collective "mo" and nominative "ga,"and finally the objective case marker "wo" (Table 3).
Table 2. Acquisition of` Japanese (Ogura, 1999)
Age

Grammatical item

1;6

Request "te" Past "ta"1;9Actor “ga” possessive “no," delimiting “mo” negative “nai” desire “tai”

2;0Citation “tte,” target “ni,” associate "to," topic "ha," polite “masu," passive, causative, potential

2;3

Means "de," source "kara," volition "yo"Table 3. Acquisition of Case Markers in Japanese (Miyata, Otomo, and Shirai, 2015)
MLU

Participle

1.5 – 1.99

Topic "ha" · modifier "no"2.0 – 2.49

Collective "mo" and nominative "ga"2.5 – 3.5

Objective case maker "wo"

2. Previous Studies
Previous studies cited on the acquisition of case markers in Korean are those of Y.J.
Kim (1997) and Zoh (1982). Y.J. Kim (1997) analyzed data on the utterances of fivechildren, saying that acquisition begins with the nominative marker "ka/i," proceedingin the order of the independent particle (delimiting) "do, " the object "nun/un," and theaccusative case marker "lul/ul." Y.J. Kim (1997) argues that the nominative "ka" is thefirst case marker that an infant uses when they begin producing binary sentences.
According to Kim (1997), the average age at which the nominative "ka" first appears
is 1;8 to 2;0, and it is used proficiently immediately after it first appears. In addition, it
is reported that the same nominative "i," which is used after a noun ending in aconsonant character, is mastered later than "ka" and there were mistakes reported
regarding the usage of "i." The independent particle (delimiting) "do" is what is
produced after the nominative "ka," appearing at almost exactly the same time as thefirst appearance of the nominative "ka." One child produced and used the nominative"ka" and the independent participle (additive) "do" for the first time at 2;0, and one
child produced the independent participle (additive) "do" (1;10) two weeks after first
producing the nominative "ka." Subsequently, it is reported that the object marker
70

"nun/un" is acquired, with individual differences apparent with the accusative "lul/ul"(It first appeared in three children between 1;11~2;3 and between 2;6~2;8 in two). It is
the final form to be acquired.
As a result of analyzing the utterances of five children, Zoh (1982) reports that they
will acquire the independent particle (delimiting) "do," nominative "ka/i," and object"nun/un" in that order. Zoh (1982) reports that independent particle (delimiting) "do"and the comitative markers-lang/-hako (“with”) were first produced before thenominative "ka," with that the locative marker-ey (“at”) mastered before the age 2;0.
In addition, they state that the object marker "nun/un" will be produced for the firsttime a few months after the first production of the nominative "ka."Y. J. Kim (1997) states that there is a slight difference in the period when thenominative "ka/i" and the independent particle (delimiting) "do" are first produced.
Therefore, to summarize the results of Kim's (1997) and Zoh's (1982) studies, it canbe said that markers will be acquired in the order of nominative "ka/i" → independent
particle (delimiting) "do" → object "nun/un" → accusative "lul/ul." However, in bothprevious studies, only the age of first production, that is, when they began to use theitem in question, is used as a criterion for mastery and attention is not paid to theinitial stage of acquisition, when children use the item in question with confidence andproductivity. In addition, when discussing the order of acquisition, it is based on theage of children, with MLU, which is said to be a measure of grammaticaldevelopment, not mentioned at all. Considering that there is a large individual
difference in the speed of language development, there is a necessity for studies toclarify the process of acquisition through an analysis that leverages MLU. In addition,
it is also necessary to conduct research that takes a comprehensive look at thedevelopmental process, that is, not only the age of first production, but right up until
the initial stage of acquisition.
Table 4. Process of Acquisition of Case Markers in Korean According to Previous Studies
Prior Research

Learning process

Y.J. Kim (1997)

Nominative "ka/i" → independent participle (delimiting) "do" → object"nun/un" → accusative "lul/ul"

Zoh (1982)

Independent particle (delimiting) "do" → Nominative "ka/i" →Accusative "nun/un"

Therefore, this study established the following two questions and developed a71

study to clarify them:
Question (1) What developmental processes are followed in the acquisition ofcase markers among first language speakers of Korean?
Question (2) Is MLU more appropriate than age as a foundation for language
development processes?
3. Research methods
3.1 The case markers that are the subject of the analysis
The case markers to be analyzed in this study are shown in Table 5. In the case of
Korean, when using a case marker, the marker will change depending on whether theword proceeding it ends with a vowel or a consonant. For example, the Koreanparticles corresponding to the nominative "ga" are ka (가) and i (이), with ka (가)used when the preceding word ends with a vowel and i (이) used when the preceding
word ends with a consonant. Therefore, there are three pairs of case markers: thenominative "ka/i," the object "nun/un," and the accusative "lul/ul." In this study, in
addition to the three case markers, we include the possessive marker "ui" and the
independent particle "do" as subjects of analysis for a total of five particles.
Table 5. The Case Markers Analyzed in This Study
Participle① Nominative "ka/i" (vowel + ka 가, consonant + i 이)
② Object "nun/un" (vowel + nun 는, consonant + un 은)③ Accusative "lul/ul" (vowel + lul 를, consonant + ul 을)④ Possessive "ui" (의)⑤ Independent Particle (Delimiting) "do" (도)Korean case markers may be omitted as in Japanese (O'Grady (1991), Lee (1989)),and syntactically, there may not be a one-to-one correspondence between thegrammatical relations of terms and their semantic roles (Kim (1990), Yang (1972)),
making them appear quite complex. However, this study focuses on the utterances ofchildren with the assumption that acquisition begins with typical usages. The
following sample sentences are utterances from the data analyzed in which a childuses a case marker.

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(1)Ka/i (nominative)엄마가 꺼 내 (JONG 1:11)

손이 아팠어 (JONG 2;1)

Emma-ka kko nae
Mom-NOM put out"Mom takes it out."

Son-i apha-sse
Hand-NOM painful-Pst1"My hand hurts."(2)Nun/un (topic)토끼는 빨리 가 (JOO 3:5)

양은 동물원 (YUN 2;7)

Ttokki-nun ppalli ka
Rabbit-TOP fast go"Rabbits go fast"

Yang-un dongmwulwuen
Sheep-TOP zoo"Sheep is a zoo"(3)Lul/ul (accusative)코를 그려요 (JOO 2;9)무엇을 읽을까? (YUN 2:5)

Kho-lul kulye-yo
Nose-ACC draw-Dec"I draw a nose"

Mues-ul ilk-ulkka?
What-ACC read-Qes"What do we read?"(4)Ui (possessive)나의 소방차 (YUN 2:5)
Na-ui sobangcha
I-POS fire engine"My fire engine"(5)Do (delimiting)이것도 있다. (JONG 1:11)
Ikes-do iss-ta
This-DEL be-Dec"There is also this."Children who have just begun to talk speak in words, without using case markers.
When they can speak in one-word and two-word sentences, their acquisition ofgrammar begins, but in the first case, it remains a sequence of words. As case
1 The abbreviations used in the glosses area as follows: ACC = accusative case marker, Dec = declarative

sentence ending, DEL = delimiting case marker, NOM = nominative case marker, POS = possessive casemarker, Pst = past tense marker, Qes = question ending, TOP = topic marker

73markers are acquired, they begin to appear in the utterances of children, but thisstudy analyzes the age of first production and the patterns of usage afterwards. In
research on the acquisition of case markers in infants, the issue of when production
first occurs is important, but the early stages of learning, when children become able
to use the items confidently and productively, is also crucial. Children become awareof grammatical elements, such as case markers, from the speech of the caregiver,and subsequently begin to use them, but even after the first time they are used, aprocess of hypothesis verification is carried out in order to learn the correct way to
use the case markers. Hypothetical verification involves the checking of the child's
own hypotheses by looking at the reactions of the caregiver to the child's speech. For
example, when a child utters a sentence that is incorrect, the parent will correct thechild’s mistake, and if there are no mistakes, the conversation will proceed, in aprocess of interaction between parent and child. As learning progresses even further,children become confident in using grammatical elements correctly, and the
proficient use of certain grammatical elements becomes apparent. At that stage, it isrecognized that acquisition has been completed to a certain extent, and in this study,
the period in which the frequency of use greatly increases is the acquisition stage.
Subsequently, the frequency of use by children gradually approaches to that of adultnative speakers.
3.2 Data Analyzed
In this study, we utilize the Ryu corpus (Ryu, 2020) available on CHILDES toanalyze the usage processes of case markers. The Ryu Corpus is a databaseconsisting of recorded natural speech that has been converted to text between threechildren (Jong, Joo, Yun) and their caregivers across 2 years, from the ages or 1 to 3
years who are acquiring Korean as a mother tongue (https://childes.talkbank.org/
access/EastAsian/ Korean/Ryu.html)). The data was recorded with a video camera ata frequency of one hour per month with a scene established in which the caregiverand the child engage in a dialogue while reading a picture book. The language datacollected totaled 81 h and 15 min of natural speech, from which 3,723 sentences inwhich the children produced the five case markers in question were analyzed as the
subjects of the study. The details of the children from whom data were collected aresummarized in Table 6.

74

Table 6. Details of the Children from Whom Data were Collected and Analyzed
Number of utte
Total
Time
Age
Sex
Siblings
number of rances subject
Recorded
to analysis
utterances
JONG 1;3~3;5

Man

None

31 h 37 min

13,764

1,451

JOO

1;9~3;10 Woman

One eldersister29 h 2 min

25,6741,264

YUN

2;3~3;9None

20 h 36 min

10,361

1,008

81 h 15 min

49,799

3,723

Man

Total
3.3 Calculations of the MLU

MLU, as proposed by Brown (1973), is an indicator of the grammaticaldevelopment of children who are acquiring English. Behind this theory is the factthat the number of sentence elements increases as the sentence structure becomes
more complicated due to the development of grammar. For example, the utterance“teddy jump” in the early stage of language acquisition is later changed to “the teddy
is jumping,” becoming longer with the addition of articles, predicates, verb
conjugation endings, etc. Brown (1973) proposes the use of morphemes to calculate
the length of utterances. In other words, in addition to independent semantic words,
conjugated endings also become units. Plural forms of nouns (-s) and verb endings(-s, -ed, and -ing) are recognized as a single morpheme each. In the above example,
teddy/ jump would be 2 morphemes and the/ teddy/ is/ jump/-ing is 5 morphemes("Mean Length of Utterances in Morphemes" (MLUm)).
In MLU, as proposed by Brown (1973), the morpheme was the unit of calculation,
but later, criticisms were voiced against MLUm. It has been pointed out that themorpheme-based MLU overestimates children's abilities in Dutch, German, andIrish. In those languages, since there are no stem-only verb forms, children areforced to use suffixed word forms from the beginning. The English language “go” is
one morpheme, but the German “geh-en,” which has the same meaning, becomes
dimorphemic with geh not used on its own. We have concluded that in suchlanguages it is better to calculate MLU by word ("Mean Length of Utterances in
Words (MLUw)”).
One may summarize the different formula for calculating MLU as follows:751) Mean Length of Utterances in Words (MLUw)
Ex) 이것도 있다. (JONG 1:11)
MLUw= 2.0 (2 words/1utterance)Ikes-do iss-ta
This-DEL be-Dec "There is also this."2) Mean Length of Utterances in Morphemes (MLUm)
Ex) 이것-도 있-다. (JONG 1:11)
MLUm= 4.0 (4 morphemes/1utterance)Ikes-do iss-ta
This-DEL be-Dec "There is also this."
The level of grammar development in Korean is also reflected by the length ofsentences. As an agglutinative language, in Korean, suffixes (mophemes) are
accumulative; therefore, the more complex the word form, the longer it becomes.
The appropriateness of the formula for calculating MLU, whether it is done through
MLU in words or MLU in morphemes, should be studied in the future as an
outstanding issue. In the case of Japanese language, one that has a high degree ofsimilarity to Korean, one study by Miyata (2012), which sought to verify theappropriateness of the MLU formula, proposed that "when using MLU, it ispreferable to calculate MLUw (Mean Length of Utterances in Words) in the earlystage of learning (up to 2 /2 and a half years of age for healthy children) andcalculate the MLUm (Mean Length of Utterances in Morphemes) when that value
exceeds 1.5.”
In this study, we conduct analyses using the "Mean Length of Utterances in Words(MLUw)." The reason for using the " Mean Length of Utterances in Words" is thatwhile the Ryu corpus (Ryu, 2020) used in this study has an enormous volume ofwords, containing scripted utterances between children and guardians, it is a corpus
for which a morphemic analysis has yet to be completed. To analyze morphemes, thecorpus must be tagged with morphemes and comments, such as the type of sentences
and the explanation of the context. Since that would be a large project that goesbeyond the scope of this study, it is our hope that a corpus, which includes amorphological analysis, will be developed in the future. However, the Ryu corpus
calculates and publishes the “Mean Length of Utterances in Words” in its data. The“Mean Length of Utterances in Words” is calculated based on "separate words" asmentioned above. The Korean language, like the English language, has thecharacteristic of each word being "separate" in its written form. What is meant byseparate words here is that there is a space between word boundaries in a sentence.
For example, taking “separate words” as a basis, the sentence “엄마가 사과를76

먹었다[Emma(mother)-ka sakwa(apple)-lul mek(eat)-essta]” (My mother ate anapple) has three different words in one utterance and an MLUw value of 3.0. In thisstudy, we used the “Mean Length of Utterances in Words,” which was currently
available, to carry out the analysis (hereafter, MLU refers to MLUw).

4. Results
The state of usage of the five case markers uttered by each child in all survey
periods is summarized in Table 7. In this study, the period when the child began to
use the case markers more than 10 times in the one hour recorded every month wasrecognized as the initial acquisition stage. There are studies that use the age of first
production as a means of judging acquisition; however, the age of first production is
not known for the data used in this study. Table 7 shows the pattern of use of casemarkers among each of the children. However, Jong and Yun were already usingcase markers at the time when the recordings began; thus, it is not clear when theyfirst began to produce them. The age at first appearance was analyzed for Joo, forwhom it was observed that the first appearance of the independent particle(delimiting) "do," was at the age of 2;2, it came at 2;7 for the object "nun/un," 2;9for the nominative "ka/i" and the accusative "lul/ul," and 3;7 for the possessive "ui."Looking exclusively at Joo, when the process of acquisition is looked at only fromthe perspective of the age of first production, the order of acquisition is as follows:
the independent particle (delimiting)"do" → object "nun/un" → nominative "ka/i"
and accusative "lul/ul" → possessive "ui." However, in this study, it is judged thatchildren acquired case markers on the basis of the initial stage of acquisition rather
than at the first age of production; therefore, the period when the use of case markers
suddenly increased by at least 10 instances is mapped through shading in Table 7.
However, as the frequency of use of "ui" is less than that of other case markers, theperiod when the frequency of use increased is shaded in, even if the number did notreach 10 or more utterances.

77

Table 7. Use of Case Markers by Each Child (token count)
JONG

1;7
1

1;8
6

ACC lul/ul

0

TOP nun/un

29

POS ui
DEL doSum
MLU

NOM ka/i

JOO
NOM ka/i
ACC lul/ul
TOP nun/un
POS ui
DEL doSum
MLU

YUN
NOM ka/i
ACC lul/ul
TOP nun/un
POS ui
DEL do
Sum
MLU

Age
2;6
23

3;5
79

SUM
token
(%)
671
46

5

4

113

8

11

40

432

30

3
2
128
3.8

21
214
1451

1
15
100

1;9
24

1;10 1;11
9
24

2;0
17

2;1
44

0

3

0

5

2

12

2

1

2

4

9

1

5

1

8

4

34

1

16

4

14

15

27

24

10

19

47

18

21

13

0
1
31
2.4

0
12
22
2.6

2
11
74
3.1

0
11
21
2.8

0
16
61
2.8

0
34
57
2.9

0
12
82
3.2

0
6
54
3.3

0
7
63
2.8

1
14
81
2.8

0
4
41
3.0

1
8
60
2.7

7
4
86
3.1

1
5
67
2.8

0
15
64
2.9

0
9
80
2.8

0
9
65
2.6

2;2
0
0
0
0
1
1
2.9

2;3
0
0
0
0
2
2
2.4

2;10
1
1
10
0
5
17
2.2

2;11
3
2
5
0
16
26
2.4

Age
3;0
11
0
18
0
9
38
2.4

3;1
17
1
34
0
23
75
2.7

3;2
12
2
65
0
43
122
2.9

3;3
16
25
69
0
12
122
2.8

3;4
44
26
54
0
13
137
3.4

3;5
99
16
60
0
25
200
3.7

3;6
82
38
48
0
16
184
3.7

3;7
73
13
17
1
10
114
3.5

3;8
42
10
31
0
23
106
3.5

3;9
7
4
16
0
9
36
3.2

3;2
12
5
7
0
5
29
3.2

3;3
16
2
14
0
3
35
3.0

3;4
17
7
27
1
2
54
3.2

3;5
60
15
35
0
20
130
3.6

3;6
37
8
27
0
16
88
3.3

3;7
43
6
20
5
4
78
3.2

3;8
72
18
35
1
19
145
4.1

3;9
45
14
21
0
3
83
3.8

2;3
2
0
2
0
2
6
2.2

2;4
2
0
3
0
3
8
2.4

2;4
0
0
0
0
1
1
2.5

2;5
2
3
3
1
3
12
2.9

2;5
0
0
0
0
0
0
2.5

2;6
2
1
9
0
0
12
2.5

2;6
0
0
0
0
0
0
2.3

2;7
3
0
32
0
5
40
2.6

2;7
0
0
1
0
2
3
2.1

2;2
31

2;8
0
0
1
0
0
1
2.2

2;8
6
1
2
0
2
11
3.5

2;3
28

2;9
2
1
2
0
4
9
2.2

2;9
13
2
13
3
17
48
3.4

2;4
40

2;10
22
3
34
2
1
62
3.0

2;5
23

2;11
17
13
24
0
1
55
3.3

Age
3;0
40
8
12
3
10
73
2.9

2;7
27

3;1
28
6
3
0
2
39
2.7

2;8
38

2;9
27

2;10 2;11
50
34

3;0
53

3;1
25

3;2
18

3;3
24

9

8

11

10

11

13

17

19

20

16

2
9
89
3.4

3
12
70
3.4

0
8
56
2.7

1
5
57
3.1

0
0
42
3.0

3;4
26

3;10
14
1
32
0
23
70
3.0

SUM
token
(%)
423
33
140
11
463
37
1
0
237
19
1264
100

SUM
(%)
token
439
44
112
11
323
32
16
2
118
12
1008
100

Note. NOM = Nominative, ACC = Accusative, TOP = Topic, POS = Possessive, DEL = Delimiting.

Looking at the results in Table 7, from the age of 1;7 to 3;5, it has been calculated
that there were 1451 sentences in which Jong used case markers during the survey
period. Although the first production of each case marker is unclear, looking at theinitial stage of acquisition, it can be seen that the age when this stage of acquisition
occurred for the topic marker "nun/un" was at the age 1;7, for the independent
marker (delimiting) "do," it was 1;8, for the nominative "ka/i," it was 1:9, for theaccusative "lul/ul," it was 2;1, and for the possessive "ui," it was 2;7. In particular,
Jong began to acquire words at an earlier stage than the other two children,
beginning to acquire case markers from the age of one year and seven months.
Jong’s MLU is also higher than the average at the age of 1 year and 7 months. He isa child who was quick to speak. There were 1264 sentences made by Joo that wereidentified as involving utterances using case markers during the survey period,
which spanned the ages of 2;2 to 3;10. The first production of each case marker was
clear, with the independent participle (delimiting) "do" first produced at 2;2, thetopic "nun/un" was first produced at 2;7, the nominative "ka/i" and the accusative"lul/ul" were first produced at 2;9, and the possessive "ui" was first produced at 3;7.
However, looking at the initial stage of mastery, we can see that they acquired the
topic "nun/un" at 2;10, the independent participle (delimiting) "do" at 2;11, thenominative "ka/i" at 3:0, the accusative "lul/ul" at 3;3, and the possessive "ui" at 3;7,
showing small differences from the order of the first production of case markers.
78

Looking at first production, the independent particle (delimiting) "do" is produced
ahead of the topic "nun/un," but in the initial stage of acquisition, the order isreversed. However, the subsequent order of the processes of acquisition was thesame for both the first production and in the initial stage of acquisition, in the orderof the nominative "ka/i", the accusative "lul/ul," and the possessive "ui". Comparedto the other two children, Joo had the slowest language acquisition. The acquisitionof case markers began when he was two years and ten months old. There were 1008
sentences in which Yun was seen to use case markers during the survey period,
which coincided with the ages of 2;3 and 3;9. Although it is not clear when eachcase marker was first produced, looking at the initial stage of acquisition, we can seethat the topic "nun/un" was first produced at 2;7, the independent participle(delimiting) "do" was first produced at 2;9, the normative "ka/i" was first produced
at 2:9, the accusitive "lul/ul" at 2;11, and the possessive "ui" at 3;7. Yun’s acquisition
was slower than Jong, but language acquisition began earlier than Joo, who began toacquire case markers from the age of 2 years and 7 months.
Table 7 contains the MLU for the period from which data were collected fromeach child. MLU tends to increase with the age of children, but it is not necessarily
directly proportionate. That is, the MLU appears to both rise and fall as the childages, but the trend is roughly for it to rise to the upper right. However, when we lookat the MLU at the time when the acquisition of case markers commences, it was 2.4
for Jong, 2.2 for Joo, and 2.6 for Yun, which can be said to be a smaller gap thanlooking at age. Therefore, it can be recognized that when MLU rises to 2.2~2.6, theacquisition of case markers begins.
Finally, Table 7 reveals the usage ratios of the five case markers. The totals areshown on the right side of the table, but the number and component ratio of the casemarkers that the child produced during the period in which they were recorded aresummarized. Jong uttered 671 sentences, including the nominative "ka/i," which
accounts for 46% of all utterances, including the five case markers. In addition, Jong
used topic "nun/un" in 432 sentences with a component ratio of 30%. They used the
independent participle (deliminating) "do" in 214 sentences (15%), the accusative"lul/ul" was used in 113 sentences (8%), and the possessive case marker "ui" in 21
sentences (1%.) It can be seen that the ratio of the use of case markers by Joo is 37%
for the topic marker "nun/un," 33% for the nominative "ka/i," 19% for the
independent participle (delimiting), "do," 11% for the accusative "lul/ul," and 0% for
the possessive "ui." Looking at the ratio of the use of case markers by Yun, the useof the nominative "ka/i" comprised 44%, the object marker "nun/un" comprised 32%,
the independent particle (delimiting) "do" comprised 12%, the use of the accusative79

object "lul/ul" comprised 11%, and the possessive case marker "ui" comprised 2%.
Taking a wider and more comprehensive perspective, the total ratio of use of thenominative "ka/i" and the object "nun/un" among the five case markers exceeds 70%,
suggesting that the nominative "ka/i" and the object "nun/un" should play importantroles in discussions of the acquisition of case markers. The nominative "ka/i," theobject "nun/un," independent particles (delimiting) "do," and accusitive "lul/ul"
comprise 10%–15%, with the possessive marker "ui" comprising the lowest
proportion, at 1%. We will consider the possessive "ui", which has a usage ratio of
1%, in the next chapter.

5. Discussion
The processes of acquiring the case markers among the three children issummarized in Table 8 below that summarizes the ages at which each child acquired
case markers as well as their MLU.
Table 8. The Process of Acquisition of Case Markers

Child
Acquisition order
JONG
TOP nun/un → DEL do → NOM ka/i→ ACC lul/ul→ POS ui
Age
1;7
1;8
1;9
2;1
2;7
MLU
2.435
2.553
3.140
3.185
3.088
JOO
TOP nun/un → DEL do → NOM ka/i → ACC lul/ul → POS ui
Age
2;10
2;11
3;0
3;3
3;7
MLU
2.238
2.352
2.350
2.752
3.473
YUN
TOP nun/un → DEL do → NOM ka/i → ACC lul/ul → POS ui
Age
2;7
2;9
2;9
2;11
3;7
MLU
2.603
3.412
3.412
3.282
3.234
From the results in Tables 7 and 8, we uncovered the following two points
regarding the acquisition process of Korean case markers.
① The processes of acquisition of case markers
Table 8 shows that each child's case marker acquisition pattern follows a constantprocess. All the children showed the same process of acquisition of case markers,
which is as follows: object "nun/un" → independent particles (delimiting) "do" →nominative "ka/i" → accusative marker "lul/ul" → possessive marker "ui."However, this result differs from the results of previous studies on Korean
80

(nominative "ka/i" → independent particle (delimiting) "do" → object "nun/un"→ accusative marker "lul/ul"). In particular, the greatest point of difference between
this study and previous studies is that in this study, the object "nun/un" is acquired at
the earliest stage. The reason for this is thought to be that the process of acquisitionchanges depending on what the criterion is used as a basis for measuring theacquisition of case markers among children. Prior studies Y.J. Kim (1997) and Zoh(1982) discuss the acquisition process based on the age when markers are first
produced. However, in this study, judgments were made based on the initial stage ofacquisition; therefore, we believe that this is the reason why the results are different
from previous studies. Considering the time when previous research was conducted,
I think it was a time when no corpus like the Ryu Corpus existed, where all speechof children and caregivers was arranged as texts, and when no linguistic analysis
systems like CLAN and CHILEDS existed. The first production of a marker may
have been immediately observable by the observer; however, a set of circumstances
may have been present whereby it was difficult for subsequent usage patterns tohave been easily produced. In addition, an MLU calculated by analyzing the scripts
of all utterances may have been impossible. In response to the question of whichcriterion is appropriate to observe the process of language acquisition, the age offirst production or the initial stage of acquisition, this study is able to adopt theinitial stage of acquisition. Looking at the results of this study, the learning processesof the three children is consistent, and we believe that this is evidence that assures
the appropriateness of using the initial stage of acquisition.
Allow us to compare the results of this study with the acquisition process ofJapanese markers. From the results of Ogura's (1999) study, when only the fivemarkers that are the subjects of this study are isolated and looking at the acquisition
processes, the following order was found: “actor ‘ga,’ possession ‘no’, delimiting‘mo’ → object ‘ha’” (Table 2). This is very different from the results of this study,
in which the topic "nun/un" was acquired before the nominative "ka/i." Ogura(1999) examined the number of people who used case markers and auxiliary verbs
more than five times in 19 minutes in a cross-sectional study of 60 people (10people every 3 months from 1;0 to 2;3). The acquisition process was summarizedbased on the point when elements of grammar used by several people and whichwere used more than five times appeared for the first time. This is a slightly different
way of studying how to longitudinally record the natural speech of a single child andobserve the first production. Although it is difficult to directly compare the methods
used in previous studies, Ogura’s (1999) study can be said to be one that is based on
the age of first production. In other words, Ogura (1999), in terms of using first81

production as a criterion of acquisition, takes the same stance as the work of Y.J.
Kim (1997) and Zoh (1982), unlike this study. This was also reflected in the research
results, with the three studies reporting the result that the nominative "ka/i" wasacquired before the object "nun/un."
However, Miyata, Otomo, and Shirai (2015) used CHILDES's Japanese corpus(five people, one hour of natural speech data per week) to investigate the acquisition
order of functional morphemes of verbs and particles. When we extract andsummarize only those case markers to be analyzed in this study, we find that theacquisition of the case markers proceeded in the order of the object "ha" andmodifier "no" → delimiting "mo" and nominative "ga" → accusative "wo" (Table3). The acquisition process is consistent with the results of this study (object"nun/un" → independent particle (delimiting) "do" → nominative "ka/i" →accusative "lul/ul" → possessive "ui"), except for the modifier "no." The Japaneseparticle "no" is different from the Korean particle "ui", in that it is used as a casemarker, also used to denote ownership, and sometimes used as an ending marker byattaching it to the end of a sentence in order to change the tone of the sentence. For
example, the “no” in “usagi no shippo” (rabbit's tail) is used as a possessive marker,
but the “no” in “tabeta no” (I have eaten “no”) is used as an ending marker to soften
the tone of the sentence and express the idea of making a soft assertion. Miyata,
Otomo, and Shirai (2015), in their results combining possessive markers and ending
markers, show that the frequency of the use of "no" is also high, showing that it isacquired at an early stage. However, because the meanings of "no" in Japanese and"ui" in Korean are different, it would be difficult to directly compare the results ofthis study with those of Miyata, Otomo, and Shirai (2015). It can, however, be said
that the frequency of the use of "no" in Japanese is certainly much higher than thatof "ui" in Korean. Please refer to Tomioka (2019) for more on the acquisition of“no” among speakers of Japanese as a first language. What I would like to note hereis that the results of this study are consistent with those of Miyata, Otomo, andShirai (2015), and that the case markers for topics were acquired at an earlier stage
than that of the nominative. Both Miyata, Otomo, and Shirai (2015) and this study
leveraged the CHILDES corpora (natural speech data) in their analysis. In addition,
Miyata, Otomo, and Shirai (2015) recognize acquisition as the time when specificelements are used in four different combinations (e.g., "This ha," "Mama ha," "I ha,"and "Cat ha"). Therefore, they are looking at frequency of use in the initial stage ofacquisition and not first production. Accordingly, the quality of the data used by
Miyata, Otomo, and Shirai (2015) is the same as this study, the analytical criteria isalso the same, and the results show the same tendencies. In other words, it can be82

said that the results of an analysis leveraging frequency of use as a basis ofacquisition and a corpus that takes the natural speech of the infant in a longitudinalmanner showed that topic case markers are acquired earlier than nominative casemarkers.
② Age and MLU in the Acquisition Process
From the results in Table 8, it was proven that there are great individual
differences in the acquisition processes and ages regarding case markers. Looking at
topic "nun/un," which was acquired first, Jong acquired it at 1;7, Yun acquired it at
2;7, and Joo acquired it at 2;10, with a one-year difference between children withquicker and slower linguistic development. Therefore, it was proven that age varies
greatly among individuals and could not be used as a criterion in the acquisition
process. However, looking at the MLUs, when acquiring the topic "nun/un," thefigures are 2.2–2.6; therefore, it can be understood that individual differences are notgreater than when looking at age. There is a difference of about 12 months in the ageof acquisition of the object "nun/un" between children who learn it quickly and those
who learn it slowly, but this falls to between 2.2 and 2.6 when viewed in terms of
MLU. MLU was found to show a smaller range of individual differences among thethree children and to be more appropriate than age as a criterion for the acquisition
process. Subsequently, taking MLU as a criterion, it becomes possible toapproximate which case marker can be acquired at what MLU. For example, thisstudy revealed that the initial stage of acquisition of the topic "nun/un" occurred at
an MLU distribution of 2.2–2.6, with this stage occurring for the final possessive
marker "ui" at an MLU distribution of 3.1–3.5. Therefore, it can be predicted that
acquisition of the topic "nun/un" will start when MLU exceeds 2.2., and that
acquisition of the possessive "ui" will commence when MLU exceeds 3.0.
While we have verified in this study that it is more appropriate to use MLU as acriterion than age, in real-life research, it is difficult to calculate MLU. Firstly,
natural speech data must be recorded over a long period of time, and the speech of
the child and the caregiver must be converted to text. Subsequently, MLU iscalculated by language processing programming (using awk programs in this study).
However, the current form of MLU refers to "MLUw," but when we try to calculatethe "morphological MLU (MLUm)," all scripts must be morphologically analyzed
with morpheme tagging information provided. In addition, language processing
programming in which the number of morphemes is counted in one sentence mustalso be developed. Regarding research into first language acquisition, as Sugiura,
Naka, Miyata, and Oshima (1997:80) pointed out, one problem in modern day
83research on language acquisition among speakers of Korean as a first language isthat "because of the enormous amount of time it takes to collect data, convert it into
text, and engage in the process of analysis, previously only a small body of researchbased on a small number of case-studies carried out by individual researchers was
possible." However, through the CHILDES (MacWinney 2000), which provides
computer-based data sharing and analytical tools, it is now possible to analyze large
volumes of natural speech data. In terms of Korean language, three corpora from
Jiwon (from 2017), Ko (from 2019), and Ryu (from 2017), have finally been madeavailable on CHILDES. However, although the scripts for all three corpora arecomplete, we are not yet been able to analyze the morphemes. It is hoped that a
project to develop morphological analysis programs will be launched in the future,that analysis of morphemes in the language used by infants will be realized, and thatmany studies leveraging CHILDES will be actively conducted.

6. Future challenges
Although the process of acquiring case markers has been clarified in this study,
there are some issues to be considered as future challenges. First, when considering
the acquisition of case markers, it is also necessary to consider the problem of theomission of case markers. In Korean, omitting case markers may not interfere withthe understanding of the sentences, depending on the context. For example, thesentence "엄마 먹어[Emma Meke] (Mother eats)" omits the nominative "ka,"which represents the subject of the sentence, but it is not difficult to grasp themeaning that "One’s mother is eating." Moreover, depending on the case marker,
there are some that are not often omitted and others that are. For example, the topic"nun/un" and the nominative "ka/i" are not frequently omitted, while the accusitive"lul/ul" and the possessive "ui" may be often omitted. This can be seen from theratio of usage of the particles shown in Table 7. The nominative "ka/i" accounted for
33–46% of case marker usage, the object "nun/un" accounted for 30–37%, theindependent particle (delimiting) "do" accounted for 12–19%, the accusative "lul/ul"
accounted for 8–11%, and the possessive "ui" accounted for 1%. Especially in thecase of the possessive "ui," looking only at the frequency of utterances, there were
only three utterances that included it during the entire survey period. The possessive''ui'' is an often-omitted particle that may be omitted without causing any issue.
Therefore, it is very rare to see the frequency of use of that marker in natural speechdata. Of course, with longitudinal spontaneous speech data there are limitations with84

the data itself, and discussing the acquisition of less frequent morphemes based onthat data may constitute a logical leap.
Secondly, as a future challenge, we believe that syntactic considerations are also
required. This study discusses the acquisition process based on frequency of use
from a morphological perspective, with the premise that "if they acquire casemarkers, children will produce them correctly." However, the acquisition of particles
is related to word order and syntax, whether the case markers were used or omitted
must be verified through a different research method, with an understanding of theword order and syntax of Korean. In that case, we believe that with natural speechdata, a hypothesis verification method through experimentation is more efficient.
This is because speech data alone cannot allow us to verify whether a childunderstands grammar and produces it correctly. It is our hope that further research
will be conducted on the acquisition of case markers.

Acknowledgments
This work was supported by JSPS KAKENHI Grant Number JP18K12384, which is
gratefully acknowledged. We wish to thank all the children and their caretakers who
participated in this study. This research would not have been possible without theircontributions.

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(Department of Foreign Language Education, Aichi Shukutoku University)
juyeonryu@gmail.com

86

韓国語教育研究（第１１号）
2021 年 9 月 15 日 発行

発行者

文 慶喆

発行所

日本韓国語教育学会

編集者

印刷所

〒577-8052 大阪府東大阪市小若江 3-4-1
近畿大学 国際学部 酒勾康裕 研究室気付『韓国語教育研究』編集委員会
金世徳、柳朱燕株式会社 仙台共同印刷