La liaison in French is a thorny subject. Some liaisons are “obligatoires” like articles and adjectives plus a noun or pronoun or another adjective (un ami; les enfants; ces activités; deux autres; deux anciens amis). Some are “facultatives” or optional, and some very important ones are “interdites”.
There are pages and pages of rules for the three categories, all requiring memorization of the rules. So for today, we will look at at least some of the “liaisons interdites”. If you can avoid saying these, you can go ahead and say all the other potential ones. You might sound overly stiff or formal, and make some liaisons that no French speaker would ever make in conversation, but at least you wouldn't be making a huge mistake.
LIAISONS INTERDITES
never after “et” (et/ on; et/ alors)
before an “h aspiré” (en/ haut; les/ héros; les/ hors d'oeuvres)
between a singular noun or proper noun and the next word (un enfant/ anglais; une question/ indiscrète;
Louis est sympathique; un cas/ intéressant;)
EXCEPT in these fixed expressions: pas à pas; accent aigu; le cas échéant; un pied-à-terre;
never before “oui (mais/ oui; il a dit/ oui)
never before “huit” and “onze” (tous les/ huit jours; dans/ onze jours)
never between the numbers “cent” and “quatre-vingt” (cent/un; cent/ onze; quatre-vingt/ un; quatre-vingt/ huit)
BUT liaison with dix-huit and vingt-huit
in an imperative between “en” or “les” and a following word (prends-en/ avec toi; fais-les/ entrer)
after a possessive or demonstrative pronoun (les miens/ aussi; ceux/ à qui je pense)
after interrogative adverbs (“quand”, “comment” and “combien” (quand/ êtes-vous né?; combien/ en avez-vous? Comment est-elle? Comment/ allez-vous au travail?)
BUT liaison after “quand” when followed by “est-ce que” or when “quand” is a conjunction (quand est-ce qu'il arrive?; Je ne sais pas quand il sera de retour; and liaison in the expression “comment allez-vous?”
never after “selon”, “vers” or “envers” (vers/ une heure; selon/ eux; envers/ elle)
after “toujours” and “bientôt” (pas toujours/ évident; toujours/ en retard)
after a pronoun ending in a nasal vowel (quelqu'un/ a dit; chacun/ a ses défauts)
EXCEPT after “on” and “en” (on est là; je voudrais en avoir;
BUT no liaison when “on” and “en” are after the verb attached by a hyphen (va-t-on/ à l'église?; donnez-en/ au garçon)
after “ils” elles” “on” in an inverted verb form (vont-ils/ écouter? Ont-elles/ eu du succès?; sont-ils/ allés?)
no liaison with “nous” and “vous” when the next word is NOT a verb (avez-vous/ une soeur? allons-nous à Paris?)