フランス語史
内的な歴史 > フランス語の基底言語と外来言語のインパクト

French is noticeably different from most other Romance languages. Some of the changes have been attributed to substrate influence—i.e. to carry-over effects from Gaulish (Celtic) or superstrate—influence from Frankish (Germanic). In practice, it is difficult to say with confidence which sound and grammar changes were due to substrate and superstrate influences, since many of the changes in French have parallels in other Romance languages, or are changes commonly undergone by many languages in the process of development. However, the following are likely candidates.
In phonology:
The reintroduction of the consonant /h/ at the beginning of a word is due to Frankish influence, and mostly occurs in words borrowed from Germanic.  This sound no longer exists in Standard Modern French (—it survives dialectally, particularly in the regions of Normandy, Picardy and Wallonia); however a Germanic h usually disallows liaison: les halles /lɛ.al/, les haies /lɛ.ɛ/, les haltes /lɛ.alt/, whereas a Latin h allows liaison: les herbes /lɛzɛrb/, les hôtels /lɛzotɛl/.
The reintroduction of /w/ in Northern Norman, Picard, Walloon, Champenois, Bourguignon and Bas-Lorrain is due to Germanic influence. All Romance languages have borrowed Germanic words containing /w/, but all languages south of the isogloss —including the ancestor of Modern French ("Central French")—converted this to /ɡw/ (which remains in some words like e.g. linguistique), which usually developed subsequently into /ɡ/. English borrowed words both from Norman French (1066 – c. 1200 AD) and Standard French (c. 1200–1400 AD), which sometimes results in doublets such as warranty and guarantee.
The occurrence of an extremely strong stress accent, leading to loss of unstressed vowels and extensive modification of stressed vowels (diphthongisation), is likely to be due to Frankish influence, and possibly to Celtic influence, as both languages had a strong initial stress. (e.g. tela -> TEla -> toile) This feature also no longer exists in Modern French.  However, its influence remains in the uniform final word stress in Modern French—due to the strong stress, all vowels following the stress were ultimately lost.
Nasalisation resulting from compensatory vowel lengthening in stressed syllables due to Germanic stress accent
The development of front-rounded vowels /y/, /ø/, and /œ/ may be due to Germanic influence, as few Romance languages outside of French have such vowels.
The lenition of intervocalic consonants (see above) may be due to Celtic influence: A similar change happened in Celtic languages at about the same time, and the demarcation between Romance dialects with and without this change (the La Spezia-Rimini Line) corresponds closely to the limit of Celtic settlement in ancient Rome. The lenition also affected later words borrowed from Germanic (e.g. haïr < hadir < *hatjan; flan < *fladon; (cor)royer < *(ga)rēdan; etc.), suggesting that the tendency persisted for some time after it was introduced.
The devoicing of word final voiced consonants in Old French is due to Germanic influence (e.g. grant/grande, blont/blonde, bastart/bastarde).
In other areas:
The development of verb-second syntax in Old French (where the verb must come in second position in a sentence, regardless of whether the subject precedes or follows) is probably due to Germanic influence.
The first person plural ending -ons (Old French -omes, -umes) is likely derived from the Frankish termination -ōmês, -umês (vs. Latin -āmus, -ēmus, -imus, and -īmus; cf. OHG -ōmēs, -umēs).
The use of the letter k in Old French, which was replaced by c and qu during the Renaissance, was due to Germanic influence. Typically, k was not used in written Latin and other Romance languages. Similarly, use of w and y was also diminished.
The impersonal pronoun on "one, you, they" – (from Old French (h)om, a reduced form of homme "man") is a calque of the Germanic impersonal pronoun man "one, you, they", reduced form of mann "man" (cf Old English man "one, you, they", from mann "man"; German man "one, you, they" vs. Mann "man").
The expanded use of avoir "to have" over the more customary use of tenir "to have, hold" seen in other Romance languages is likely to be due to influence from the Germanic word for "have", which has a similar form (cf. Frankish *habēn, Gothic haban, Old Norse hafa, English have).
The increased use of auxiliary verbal tenses, especially passé composé, is probably due to Germanic influence. Unknown in Classical Latin, the passé composé begins to appear in Old French in the early 13th century after the Germanic and the Viking invasions. Its construction is identical to the one seen in all other Germanic languages at that time and before: « verb "be" (être) + past participle » when there is movement, indication of state, or change of condition; and « "have" (avoir) + past participle » for all other verbs. Passé composé is not universal to the Romance language family—only Romance languages known to have Germanic superstrata display this type of construction, and in varying degrees (those nearest to Germanic areas show constructions most similar to those seen in Germanic). Italian, Spanish and Catalan are other Romance languages employing this type of compound verbal tense.
The heightened frequency of si ("so") in Old French correlates to Old High German so and thanne
The tendency in Old French to use adverbs to complete the meaning of a verb, as in lever sus ("raise up"), monter amont ("mount up"), aler avec ("go along/go with"), traire avant ("draw forward"), etc. is likely to be of Germanic origin
The lack of a future tense in conditional clauses is likely due to Germanic influence.
The reintroduction of a vigesimal system of counting by increments of 20 (e.g. soixante-dix "70" lit. "sixty-ten"; quatre-vingts "80" lit. "four-twenties"; quatre-vingt-dix "90" lit. "four-twenty-ten") is due to North Germanic influence, first appearing in Normandy, in northern France. From there, it spread south after the formation of the French Republic, replacing the typical Romance forms still used today in Belgian and Swiss French. The current vigesimal system was introduced by the Vikings and adopted by the Normans who popularised its use (cf Danish tresindstyve, literally 2 times 30, or 60; English four score and seven for 87)[要出典]. Pre-Roman Celtic languages in Gaul also made use of a vigesimal system, but this system largely vanished early in French linguistic history or became severely marginalised in its range. The Nordic vigesimal system may possibly derive ultimately from the Celtic. Old French also had treis vingts, cinq vingts. (cf. Welsh ugain "20", deugain "40", pedwar ugain "80" lit. "four-twenties").
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