When do you use etre to form the perfect tense of a verb?

There are a number of verbs which take etre as the auxiliary verb when forming the perfect tense. These are generally verbs showing movement or a change of state and a useful technique to remember them is MRS VANDERTRAMP, where each letter represents each verb. M=monter, R=retourner, S=sortir, V=venir, A=arriver, N=Naitre, D=descendre, E=entrer, R=rester, T=tomber, R=rentrer, A=aller, M=mourir, P=partir. These verbs also need the past participle to agree with the subject of the sentence. For instance you would say "elles sont parties" - "they(female, plural) left". The past participle "parti" changes to "parties" because the subject is feminine and plural.