Un tréma is this sign ¨ that you find mainly over vowels e, i + u, over vowels u + y in last names/locations and over vowels a, o + u in borrowed words. It has three functions : #1. It indicates the necessity of pronouncing separately two graphemes instead of treating them like a couple. #2. It makes a vowel mute. #3. It symbolises an umlaut (germanic sign).
Nouns : Noël (Christmas, m), ciguë (hemlock, f), canoë (canoe, m), oïl (former name of northern/central France, bits of Belgium + Switzerland), skaï (old pu leather from the 80s), raï (north african music), ouïe (hearing, f), caïd (mob boss, m), coït (coitus, rare), païen-ne (pagan), laiüs (long annoying speech, patter, m), aïeux (forefathers), aïeul-e (grandparent), caïman (cayman, m), bonsaï (bonsai, m), naïveté (naivety, f), laïcité (secularity, f), héroïne (female heroin of a novel, drug, f), glaïeul (gladiolus, m), faïence (earthenware, f), égoïsme (selfishness, m), héroïsme (heroism, m), ambiguïté (ambiguousness, f), coïncidence (coincidence, f), dadaïsme (dadaism, m), Aïd-El-Kebir (Eid al-Adha), paranoïa (paranoia, f),
Adjectives : aiguë (high-pitched, sharp (pain), f), exiguë (cramped, tiny, f), ambiguë (ambiguous, f), contiguë (adjacent to, f), naïf/naïve (naive), laÏc/laïque (secular, religion-neutral), haï-e (hated), inouï-e (incredible, never heard of before), ovoïde (egg-shaped, ép), Hawaïen-ne (Hawaiian), égoïste (selfish, ép), stoïque (stoical, ép), héroïque (heroic, ép), hébraïque (hebraic, hebrew, ép), archaïque (archaic, ép),
Verbs : ouïr (to hear, old/rare), haïr (to hate), laïciser (to secularise),
Others : ô!, aïe (ouch!), taïaut (hunt vocabulary, fam means ‘go!’ or ‘run!’), par ouï-dire (by hearsay, invariable)