Diacritics
French makes use of five diacritics that can modify certain letters. Unlike in some languages, letters with diacritics are not considered to be separate letters; for example, the diacritics are ignored when alphabetizing a list of words. Further, the diacritics are often omitted from capital letters (with capital ⟨é⟩ being written as ⟨E⟩, and so on), though strictly speaking, this is not considered correct, (see Accentuation des majuscules, on the website of l’Académie française) as it is merely a remnant of non-computerized printing that could not accommodate accented capital letters.
There are no letters that always need a diacritic to produce a given sound; rather, where a given letter has multiple possible sounds, a diacritic indicates that the sound that might be expected from the context is not the one that is used.
The acute accent (l'accent aigu) ⟨´⟩ is used only on ⟨e⟩. It normally indicates that the vowel represents /e/, as in épaule ('shoulder') and détail ('detail'), when ⟨e⟩ without an accent might represent /ɛ/ or /ə/. In certain syllables, however, ⟨é⟩ is written while the pronunciation in most dialects is /ɛ/, as in céderai ('I will give up') and réglementaire ('regulatory'); the 1990 spelling reform (see below) declared that in these cases, ⟨è⟩ should be used instead, but this spelling reform is not yet widely adopted.
The grave accent (l'accent grave) ⟨`⟩ is used on ⟨e⟩ to indicate that it represents /ɛ/, as in manière (way) and après (after), when it is followed by a single consonant. It is also used on ⟨a⟩ and ⟨u⟩ to distinguish certain homophones, such as ou ('or') and où ('where'), and la ('the') and là ('there').
The cedilla (la cédille) ⟨¸⟩ is used only on ⟨c⟩, and only when it is followed by ⟨a⟩, ⟨o⟩, or ⟨u⟩; it indicates that the ⟨c⟩ represents a soft /s/ (the pronunciation that ordinarily occurs before ⟨e⟩, ⟨i⟩, and ⟨y⟩) rather than /k/.
The circumflex (l'accent circonflexe) ⟨ˆ⟩ can be used on any of the letters ⟨a⟩, ⟨e⟩, ⟨i⟩, ⟨o⟩, and ⟨u⟩. It does not indicate a different pronunciation of ⟨i⟩ or ⟨u⟩, and does not affect the pronunciation of ⟨a⟩ in most dialects (though in some, it roughly indicates a contrast between /a/ and /ɑ/). On ⟨o⟩, it indicates /o/ (contrasting with /ɔ/), and on ⟨e⟩, it indicates /ɛ/, duplicating the function of the grave accent. The circumflex is chiefly of historical relevance; it was added in the 18th or 19th century to certain words with an unpronounced ⟨s⟩ (e.g. forest → forêt) or another unpronounced letter (e.g. aage → âge).
The diaeresis (le tréma) ⟨¨⟩ shows that two vowels are pronounced separately (i.e., that the vowel pair belong to separate syllables), compare the forms of the verb haïr /a.iʁ/ ('to hate'): je hais ('I hate') /ɛ/, nous haïssons ('we hate') /a.i.sɔ̃/). It is normally written on the second vowel. It is also added above the feminine adjectival ending -e when the masculine form ends in -gu: aigu, ambigu → aiguë, ambiguë. The same practice is not followed, however, for verbs whose stem ends in -gu. For example, the forms of the verb arguer are exact visual rhymes with those of targuer, even though the two verbs are pronounced very differently (/aʁɡɥe/ vs. /taʁɡe/, j'argue /ʒaʁɡy/ vs. je targue /ʒətaʁɡ/).
Read more about this topic: French Orthography