When do you use 'avoir' and 'être' for past tenses?

In the French perfect tense, avoir and être are both used as auxiliary, or helping, verbs, like have in I have eaten in English. They are placed before the verb which shows meaning: j'ai mangé or il est allé. The usual auxiliary verb is avoir, but in some verbs, être is used. These can be verbs of movement (aller, venir, descendre, mourir, naître) or reflexive verbs like se lever: je me suis levé. The good thing is that you can just learn which auxiliary goes with which verb when you learn the verb, and most of the time it will be avoir.