Question

What has to happen to use être instead of avoir for the formation of passé composé?

Answer 1

There are two types of passé composé in French, the verbs that use être and the verbs that use avoir. Avoir and être are called auxiliary verbs or les verbes auxiliares. Avoir is used is most verbs, but there are a few exceptions. The exceptions are called DR & MRS VANDERTRAMP. This is an acronym for the 16 exceptions.

Devenir
Revenir
&
Monter
Rester
Sortir

Venir
Aller
Naître
Descendre
Entrer
Rentrer
Tomber
Retourner
Arriver
Mourir
Partir

Every single reflexive verb also uses être. These are verbs that in the infinitive form are "se", like se coucher, s'embrasser, and, se prendre.

Answer 2

To add to the answer besides mine, you have to have noun-verb tense agreement, which means if the verb is feminine, an e is added to the end of the verb which is conjugated in the passé composé (NOT the helping verb). If it is plural, an s is added at the end of that same verb. Feminine and plural adds an es at the end of that verb. Hope this is helpful to you