When the Misidentified Adverbial Phrase Functions as a Complement (2024)

Abstract: This study investigates the predicate-argument structure in Korean language processing. Despite the importance of distinguishing mandatory arguments and optional modifiers in sentences, research in this area has been limited. We introduce a dataset with token-level annotations which labels mandatory and optional elements as complements and adjuncts, respectively. Particularly, we reclassify certain Korean phrases, previously misidentified as adverbial phrases, as complements, addressing misuses of the term adjunct in existing Korean treebanks. Utilizing a Korean dependency treebank, we develop an automatic labeling technique for complements and adjuncts. Experiments using the proposed dataset yield satisfying results, demonstrating that the dataset is trainable and reliable.

## introduction research on the predicate-argument structure of natural languages has been at the center of the studies of syntax and semantics. the relations between the predicate and its arguments in a sentence or a clause not only reveal the type of arguments the particular predicate needs syntactically, but also shed light on the possible semantics the predicate can provide. in the field of natural language processing (nlp), tasks such as syntactic parsing and semantic role labeling (srl) largely rely on some assumptions and groundwork concerning the predicate-argument structure to understand which arguments play an essential role in the clause, and which arguments do not. despite the importance of understanding the predicate-argument structure and determining whether a syntactic daughter foot_0 of a predicate appears to be mandatory or optional, studies on differentiating core and non-core argument candidates foot_1 have been lacking. this is largely due to the fact that such datasets are not necessarily associated with any specific nlp task directly. on the other hand, both linguistics and nlp can benefit from a dataset that clearly marks the core and non-core syntactic daughters dependent on the predicate in korean. moreover, whether a syntactic daughter is mandatory or optional can serve as an additional feature during syntactic and semantic processing of the korean language and boost the performance of the models on related nlp tasks. unfortunately, there is no dataset with the aforementioned annotation for the korean language, and no method that labels classified syntactic daughters has been proposed. korean is an agglutinative language that follows a subject-object-verb (sov) word order. one of the major characteristics that differentiates korean from many other non-agglutinative languages is its extensive use of postpositions as case markers. postpositional markers are suffixes that adhere to the root of the words, and they are functional morphemes that modify the semantics of the stem lexeme. postpositions of arguments are usually morphological cases, which are externalizations of the corresponding grammatical cases the arguments carry. on the other hand, @xcite argue that some oblique markers do not indicate any semantic role. rather, they present semantic and pragmatic information given a construction. @xcite further investigate korean postpositions under the framework of categorial grammar, suggesting that not all korean postpositions mark the case. in this study, we present a corpus with token-level annotations revealing the mandatory and optional elements in the predicate-argument structure. to the best of the authors' knowledge, this is the first study attempting to reclassify certain phrases in korean, previously misidentified as adverbial phrases foot_2 and modifiers, into complements, in order to address the misuse of the term adjunct in existing korean treebanks. we follow the binary distinction based on the dictionary definition and clearly define and distinguish the core and non-core argument candidates as complements and adjuncts, respectively, and demonstrate the linguistic rationale behind our proposed binary distinction. we utilize an existing korean dependency treebank, and develop a novel technique for the automatic generation of complement and adjunct labels onto the sentences in the treebank. a dataset is therefore constructed with all complements and adjuncts for the verbal predicates in the treebank's sentences annotated. we further illustrate different levels of korean predicate-argument structures by analyzing some converted data from the treebank, and train sequence labeling models using the constructed dataset. experiment results suggest that our dataset is trainable and reliable. ## linguistic debates for several decades, discussions have unfolded around the characteristics and semantic functions of arguments and modifiers within the realm of generative grammar. linguists who utilize structures such as principles and parameters (p&p) @xcite or head-driven phrase structure grammar (hpsg) @xcite ) make a tripartite distinction @xcite . they argue for the uniqueness of the specifier (an immediate subordinate of the phrase, often the subject) and the complement (the counterpart of the head, typically the object) within a verb phrase, apart from the adjuncts (optional and serve to refine the head's meaning). categorial grammar (cg) @xcite gives an alternative analysis of the predicate-argument structure. a binary distinction is made between elements associated with the predicate: the mandatory complements that complete the meaning of their head and the optional adjuncts that offer further semantics to the head's meaning @xcite . what p&p and hpsg categorize as both 'specifier' and 'complement' are collectively termed as 'complement' under categorial grammar. in our study, we utilize the manually-created subcategorization frame information and make a binary distinction between mandatory complements (i.e., arguments) and optional adjuncts (i.e., modifiers) within the predicate-argument structure of korean. complements are syntactically obligatory elements that serve as arguments at the semantic level, while adjuncts are syntactically optional and function as modifiers semantically. complements and adjuncts can be distinguished through the application of a relativization test, wherein complements can function as the head of a relative clause, while adjuncts cannot @xcite . another criterion has been suggested by utilizing particle-elimination wherein complements can occur without case makers, while adjuncts cannot @xcite . previous korean treebanks classify all phrases with non-subject and non-object postpositions as adverbial and categorize them as adjuncts, whereas by strictly following the distinction mentioned above, it becomes evident that some of these so-called 'adverbial phrases' are selected by the predicate and should therefore be considered complements. specifically, adverbial phrases, when correctly identified in their syntactic function, always act as adjuncts and never as complements. to better understand the possible arguments of korean verbs, our study utilizes the sejong dictio-nary. this dictionary is part of the sejong corpus, coordinated by the national institute of korean language. the sejong dictionary is organized such that for each verbal predicate in korean, all possible senses and their detailed syntactic and semantic information are provided (appendix a.2). specifically, we rely on the subcategorization frames of the verbal predicate. it is worth noting that the subcategorization frame encompasses the required arguments alongside their postpositions. extraction of complements and adjuncts a heuristic method is implemented for the extraction of complements and adjuncts from the klue dependency treebank, based on existing annotations of dependency relations in the treebank and the sejong dictionary. for an annotated sentence from the treebank, the method first locates all possible verbal predicates in the sentence, based on the xpos tag of the token and whether the canonical form of the verbal component of the token has its corresponding entry in the sejong dictionary. for each verbal predicate that is found, the whole dependency tree is traversed to find the constituents that serve as its daughters. following this, the heuristic method refers to the subcategorization frames of this verb from the sejong dictionary, and for each frame, it compares the postpositions of the arguments specified in the frame and the postpositions of the constituents that are daughters of the verb. those constituents whose postpositions can be found in the frames are considered complements, whereas the daughters whose postpositions are not included in the frame arguments are classified as adjuncts. the only exceptions are the syntactic daughters of the verb which do not have postpositions. in these cases, they are excluded from the aforementioned binary distinction, and are neither counted as complements nor counted as adjuncts. subsequently, the frame(s) that best fit the daughters of the verbal predicate are returned. examples of projected sentences are presented in appendix b, and sequence labeling experiments on the constructed dataset are conducted in appendix c. ## data analysis we count the numbers of instances that exactly match, partially match, or do not match the frame arguments, based on the extracted adjuncts and complements of the predicates. one of the most common causes of case (1) is noun phrase ellipsis (np-ellipsis), in which nouns or noun phrases get elided. in figure 1b , the subject of the clause is a ssi-neun ('mr. a') which is shared with the preceding clause as shown in figure 1a . the latter subject is therefore elided, resulting in a mismatch where the subject, as suggested by the frame, cannot be located in the clause. for case (2), instead of being a complement, an argument candidate may be an adjunct, as long as it cannot be located in the frame. figure 1c serves as a good example, where its subject and object are paired with the corresponding arguments in the frame and are complements, but the remaining predicate: 제출하다 (jechulhada 'to submit') type: perfect match frame: @xmath0: predicate: 마치다 (machida 'to finish') type: partial match, all arguments in the frame are paired frame: @xmath1: ... a significant distinction is that some seemingly 'adverbial' phrases are now considered complements rather than adjuncts. figure 1a features three arguments, including the subject, the object, and the dative gyeongchal-e ('to the police'). while the subject and the object are considered complements indisputably, the phrase gyeongchal-e is considered an adjunct in the treebank. however, the subcategorization frame from the dictionary suggests that the phrase, carrying a dative case with the postposition e, is a required argument of the predicate and should therefore be considered a complement, as marked in figure 1a . 5 it is worth mentioning that we consider adverbs and clausal constructions neither complements nor adjuncts, and excluded them from the list of argument candidates. they usually lack postposition markers and do not contribute to the semantics of the predicate in a way complements and adjuncts do. as figure 1c suggests, the adverb jomangan ('soon') is not a valid argument candidate and is not marked as either a complement or an adjunct by the proposed extraction method. within the 32,223 sentence-frame pairs where a match, either perfect or partial, is guaranteed, 21,155 complements and 6,757 adjuncts are found. this suggests an overwhelmingly larger portion of complements compared to adjuncts in korean. ## related work there have been numerous linguistic resources constructed for the purpose of syntactic and semantic analyses. among them, dependency treebanks provide syntactic information in dependency grammar, and they are usually utilized for dependency parsing tasks in natural language processing. universal dependencies (uds) @xcite is designed to ensure consistent labeling of grammatical structures, such as parts of speech, morphological features, and syntactic dependencies, across different languages. for korean, annotation guidelines have been proposed for the construction of korean ud treebanks @xcite . three different constituency treebanks, namely the kaist treebank @xcite , the sejong treebank, and the penn korean treebank @xcite , have been proposed for the korean language. however, only the sejong and the penn korean treebanks represent constituents of a clause, including subjects and objects. this is facilitated by the distinction made possible by nominative and accusative postpositions in korean. a noun phrase that ends with an adverbial postposition, such as a dative, locative, or temporal phrase, is categorized as either an adjunct (-ajt) or a complement (-comp) in both the sejong and korean penn treebanks. although -comp in the penn korean treebank, distinct from subjects (-sbj) or objects (-obj), signifies a complement, its function aligns more closely with that of an adjunct. several resources of dependency parsing data for korean have been made available in the past, including the gsd treebank @xcite , the kaist dependency treebank @xcite , the penn korean universal dependency treebank foot_5 @xcite , and the korean language understanding evaluation (klue) benchmark dependency parsing dataset @xcite . semantic analyses of the korean language benefit from the task of srl. @xcite adapt and conduct srl on korean, and @xcite introduce annotation strategies for korean srl with a constructed dataset. however, none of the previous work has pointed out that some so-called 'adverbial phrases' are misidentified and should be classified as complements, diverging from the perspectives of syntacticians and semanticists. ## conclusions in this study, we refine the distinction between core and non-core argument candidates, namely complements and adjuncts, in korean. a binary distinction based on the linguistic features of korean is described herein, and we propose and implement an effective method to automatically generate the labels of complement/adjunct. data from a dependency treebank is leveraged to construct a corpus with annotations indicating both complements and adjuncts of all possible verbal predicates from the sentences in this treebank. sequence labeling models trained on the proposed dataset give satisfactory results, indicating that the constructed dataset is of good quality. we believe that the dataset can serve as supplementary data in both linguistics and nlp. the proposed method would save the labor of the linguists such that they could focus on analyses without sacrificing time on finding examples with specific predicate-argument structures. the dataset is also suitable for training language models for a better understanding of the korean sentence structure, which can benefit downstream tasks. 7 limitations given the comprehensive nature of this study, we believe there are no significant limitations that need to be acknowledged to provide context for the findings or to guide future research efforts.