A small number of words ending in a nasal vowel show liaison by inserting /n/ when followed by a word that starts with a vowel. Two scenarios exist:
a) the nasal vowel changes to oral and a nasal consonant is inserted. Words of this category include pre-nominal adjectives like ancien [[ãsjɛ̃] , moyen [mwajɛ̃] , prochain [pʁɔʃɛ̃] and bon [bõ] , e.g.:
l'ancien élève [lãsjɛnelɛv]
le moyen âge [ləmwajɛnaʒ]
le prochain enfant [ləpʁɔʃɛnãfã]
bon anniversaire [bɔnanivɛʁsɛʁ/]
b) the vowel remains nasal and a nasal consonant is inserted; words of this category include on [õ], en [ã], aucun [okœ̃], un [œ̃], bien [bjɛ̃], rien [ʁjɛ̃], mon [mõ], ton [õ], son [sõ], e.g.:
on est [õnɛ]
en été [ãnete]
un homme [œ̃nɔm]
rien à faire [ʁjɛ̃nafɛʁ]
mon ami [mõnami]
Note that while this is the general trend, some speakers do denasalize the vowel in the words mon, ton and son.
As in other cases of liaison, the inserted /n/ belongs to the following syllable, e.g.: [mõ - na - mi]
BonPatron