When is the passé composé used and when do I use avoir or être?

The passé composé is used to describe completed or repeated actions that were done in the past. It is translated into English as either the past simple tense (e.g. I ate) or the present perfect tense (e.g. I have eaten). The passé composé is a compound tense. This means it is formed using two parts; an auxiliary (avoir or être) conjugated in the present tense, and the past participle of the main verb you want to conjugate. Whether you use avoir or être is determined on the verb you are conjugating. Most verbs will take avoir. However, a select few will take être as their auxiliary. These can be remembered using the well-known acronym MRS VANDERTRAMP. M=monter, R=rester, S=sortir, V=venir, A=aller, D=descendre, E=entrer, R=retourner, T=tomber, R=rentrer, A=arriver, M=mourir, P=partir. As well as all the verbs built on these verbs, e.g. repartir, devenir, rentrer, etc..