French grammar is sometimes trumped by pronunciation, as in the case of euphonic adjectives. Because French does not like the hiatus created when a word ending with a vowel precedes a word that begins with a vowel or mute h, a few adjectives change their spelling—and thus their pronunciation—for purely euphonic reasons.
Some liaisons are forbidden: they aren't—can't be—pronounced even in the most formal French. Sometimes this is to avoid confusion with similar expressions, sometimes it has to do with a sort of respect for names and foreign words, and other times there's no apparent logic to it.
In English, we say that the vowels are "a, e, i, o, u, and sometimes y." The last couple of words hint at one of the keys to understanding pronunciation: a vowel is not so much a letter as the sound represented by a letter or combination of letters.