How do I know if verbs take "avoir" or "être"?

The difference between avoir and être is a grammatical distinction that often confuses students, but is absolutely necessary to know when forming the perfect tense, as well as other more complex tenses such as the future conditional and pluperfect. The majority of verbs take avoir, but thankfully, there is a handy acronym that you can use in order to remember the odd few that are always used with être.

M - Mourir (to die)

R - Retourner (to return)

S - Sortir (to go out)

V - Venir (to come)

A - Arriver (to arrive)

N - Naître (to be born)

D - Descendre (to go down)

E - Entrer (to enter)

R - Rester (to stay)

T - Tomber (to fall)

R - Rentrer (to go back in)

A - Aller (to go)

M - Monter (to go up)

P - Partir (to leave)

Make a big poster with the MRS VANDERTRAMP acronym, and gradually as you revise it will be become easier to recall these few verbs to mind more easily, and make sure you always form tenses with the correct auxiliary verb.