フランス語史
NOTE: Changes here affect oral and nasal vowels alike, unless otherwise indicated.
/o/ > /u/.
/l/ before consonant becomes /w/.
/ue/ and /eu/ > /œ/.
Rising diphthongs develop when first element of diphthong is /u/, /y/ or /i/, causing the stress to shift to the second element in these cases (hence /yi/ [yj] > [ɥi]).
/oi/ > /we/. This in turn develops to /ɛ/ in some words, e.g. français; note doublet François.  Much later, perhaps in the 17th century, remaining /we/ sounds > /wa/ except in "court" pronunciation. (The /wa/ pronunciation was then stigmatized as "vulgar" until the French Revolution but remaining more or less in use in Quebec.) However, nasalized /wẽ/ was unaffected; hence ModF 'coin' "corner" /kwɛ̃/ not **/kwɑ̃/.
/ai/ merges into /ɛ/; after this, 'ai' is a common spelling of /ɛ/, regardless of origin. ('è' is a later development.)
/e/ merges into /ɛ/ in closed syllables.
/ts/ > /s/, /tʃ/ > /ʃ/, /dʒ/ > /ʒ/.
Loss of /s/ before any consonant, with lengthening of preceding vowel.  This may have begun as early as 900 AD or so, when /s/ before a consonant became /h/.  Later on the /h/ vanished with compensatory lengthening of the preceding vowel.  From borrowings into English, it appeared that this latter stage had already occurred in Old French when the following consonant was voiced but not when it was unvoiced.  By the end of Old French, the latter stage was complete and a whole new set of phonemically lengthened vowels developed.  These were still marked in writing with an 's', but starting around 1700 were marked instead with circumflex over the vowel (perhaps because actual pronounced /s/ had been reintroduced into that position in certain words, e.g. due to borrowing of learned words from Latin.)
Development of two low vowels /a/ and /ɑ/.  The latter was initially an allophone of /a/ that occurred before /s/ and /z/, and become phonemic when /ts/ merged with /s/. (e.g. Mod. Fr. 'chasse' /ʃas/ "(he) hunts" < */cattsa/ < captiat vs. 'châsse' /ʃɑs/ "reliquary, (eyeglass) frame" < */cassa/ < capsa "strong box".) Later losses of /s/ produced further minimal pairs, e.g. 'pâte' /pɑt/ "paste" < VL *pasta vs. 'patte' /pat/ "paw" < VL *patta; or 'bas' /bɑ/ "low" < /bas/ < bassum vs. 'bat' /ba/ "(he) beats" < /bat/ < VL *battet < battuet.)
NOTE: Changes here affect oral and nasal vowels alike, unless otherwise indicated.
/au/ > /o/.
/ei/ > /ɛ/.
Loss of final consonants before a word beginning with a consonant.  This produces a three-way pronunciation for many words (alone, followed by a vowel, followed by a consonant), which is maintained to this day in the words 'six' "six" and 'dix' "ten" (and until recently 'neuf' "nine"), e.g. 'dix' /dis/ "ten" but 'dix amis' /diz ami/ "ten friends" and 'dix femmes' /di fam/ "ten women".
(Around this time, subject pronouns become mandatory.)
(fill in further)
初期現代フランス語(1700年代)まで
音素的に長音化したほとんどの母音の喪失。Standing alone語における語末子音の喪失。このことは多くの単語に対して二様の発音を可能にし (次の単語が母音で始まるか否かに密接に関係)、たとえば、 'nous voyons' /nu vwajɔ̃/ "we see"  'nous avons' /nuz avɔ̃/ "we have" など、今日に至るまで維持されていることが多い。この現象はリエゾンとして知られる。
'oi' /we/ > /wa/ (上記を参照 – Through late Old French) or /ɛ/ (e.g. étoit > était – 19th c.).
(fill in further)
/r/ becomes uvular sound: trill /ʀ/ or fricative /ʁ/, (replacing the rolled 'r' formerly often used by the clergy).
Loss of final /ə/.  Loss of /ə/ elsewhere unless a sequence of three consonants would be produced (such constraints operate over multiword sequences of words that are syntactically connected).
Gradual loss of liaison
Gradual loss of the "ne" in negations, "je n'ai pas" becomes "j'ai pas".
(fill in further)
Progressive nasalization of vowels before /n/ or /m/ occurred over several hundred years, beginning with the low vowels, possibly as early as c. 900 AD, and finished with the high vowels, possibly as late as c. 1300 AD.  Numerous changes occurred afterwards, continuing up through the present day.
The following steps occurred during the Old French period:
Nasalization of /a/, /e/, /o/ before /n/ or /m/ (originally, in all circumstances, including when a vowel followed).
Nasalization occurs before, and blocks, the changes /ei/ > /oi/ and /ou/ > /eu/.  However, the sequence /ɔ̃i/ occurs because /oi/ has more than one origin, e.g. 'coin' "corner" < cŭneum.  The sequences /iẽn/ or /iẽm/, and /uẽn/ or /uẽm/, also occur, but the last two occur in only one word each, in each case alternating with a non-diphthongized variant: 'om' or 'uem' (ModF 'on'), and 'bon' or 'buen' (ModF 'bon').  The version without the diphthong apparently arose in unstressed environments and is the only one that survived.
Lowering of /ẽ/ and /ɛ̃/ to /ã/; but unaffected in the sequences /jẽ/ and /ẽj/ (e.g. 'bien', 'plein').  The merging of /ẽ/ and /ã/ probably occurred during the 11th  or early 12th century, and did not affect Old Norman or Anglo-Norman.
Nasalization of /i/, /u/, /y/ before /n/ or /m/.
The following steps occurred during the Middle French period:
Lowering of /ũ/ > /õ/ > /ɔ̃/. (Note that most /ũ/ come from original /õ/, as original /u/ became /y/.)
Denasalization of vowels before /n/ or /m/ followed by a vowel or semi-vowel. (Note that examples like 'femme' /fam/ "woman" < OF /fãmə/ < fēmina and 'donne' /dɔn/ "(he) gives" < OF /dũnə/ < dōnat, with lowering and lack of diphthongization before a nasal even when a vowel followed, prove that nasalization originally operated in all environments.)
Deletion of /n/ or /m/ after remaining nasal vowels (i.e. when not protected by a following vowel or semi-vowel). Hence 'dent' /dɑ̃/ "tooth" < */dãt/ < OFr 'dent' /dãnt/ < EOFr */dɛ̃nt/ < dĕntem.
The following steps occurred during the Modern French period:
/ĩ/ > /ẽ/ > /ɛ̃/ > [æ̃].  This also affects diphthongs such as /ĩẽ/ > /jẽ/ > /jɛ̃/, e.g. 'bien' /bjɛ̃/ "well" < bĕne; /ỹĩ/ > /ɥĩ/ > /ɥɛ̃/, e.g. 'juin' /ʒɥɛ̃/ "June" < jūnium; /õĩ/ > /wẽ/ > /wɛ̃/, e.g. 'coin' /kwɛ̃/ "corner" < cŭneum.  Note also /ãĩ/ > /ɛ̃/, e.g. 'pain' /pɛ̃/ "bread" < panem; /ẽĩ/ > /ɛ̃/, e.g. 'plein' /plɛ̃/ "full (m.s.)" < plēnum.
/ã/ > /ɑ̃/.
/ỹ/ > /œ̃/.  In the 20th century, this sound has low functional load and has tended to merge with /ɛ̃/.
This leaves only four nasal vowels /ɛ̃/, /ɑ̃/, /ɔ̃/, and /œ̃/, and increasingly only the three /ɛ̃/, /ɑ̃/, /ɔ̃/.
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.