Avoir or être in the perfect tense?

French verbs either take the auxiliary verbs avoir or être when conjugating in the perfect tense (or passé composé). The best way to remember which verbs take which auxiliary is the MRS VANDERTRAMP method - the verbs which fall into this method all take être (there are of course some exceptions/additions to this rule, but generally, this works!). Monter Retourner Sortir Venir Arriver Naître Descendre Entrer Rester Tomber Rentrer Aller Mourir Partir.

Reflexive verbs like se laver, s'habiller etc also take être in the perfect tense.

When conjugating être verbs in the perfect tense, make sure you remember to make your agreements! For example - il est allé BUT elle est allée Ils sont descendus BUT Elles sont descendues.