When do you use avoir and when do you use être in the past tense?

Most verbs use avoir, but there are a specific group of verbs that take être as their auxiliary verb in not just the past tense, but in all of the compound tenses (such as the pluperfect tense).

A nice way to remember this verbs is using MR VANSTRAMPED (there are variations of this that you may know):

Monter

Rester

Venir

Arriver

Naître

Sortir

Tomber

Retourner

Aller

Mourir

Partir

Entrer

Descendre

Also, verbs that are reflexive must also take être, for example se laver (to wash oneself) e.g. je me suis lavé(e)

All the verbs that take être must agree with their subject - if the subject of the verb is masculine or feminine, singular or plural!