How do I know when to use être or avoir in the passé composé?

To form the passé composé, you need three parts. The pronoun of who or what is doing the verb e.g Je. Then we need an auxiliary verb. An auxiliary verb is the first part of two verbs we use. The auxiliary verb can be either être or avoir in the present tense. There is a simple way to remember which verbs take être: DR & MRS VAN DE TRAMPP.

Descendre, Rentrer, Mourir, Rester, Sortir, Venir, Arriver, Naître, Devenir, Entrer, Tomber, Retourner, Aller, Monter, Partir, Passer

Most of these verbs are verbs of motions or changing of state, condition or place e.g moving up, moving down, going somewhere etc. Reflexive verbs also take être. All other verbs take avoir as its auxiliary verb.