When do I have to use être and when do I have to use avoir with the perfect tense?

Être should be used with MRS VAN DER TRAMP verbs (Monter, Retourner, Sortir, Venir, Arriver, Naître, Descendre, Entrer, Rester, Tomber, Rentrer, Aller, Mourir, Partir) and any reflexive verbs (s'habiller, se lever, se laver). Avoir is used with all other verbs. An easy way to remember MRS VAN DER TRAMP verbs is to think of them as verbs of movement and change. If we were to conjugate monter in the perfect tense with the first person singular it would be: je suis monté(e) If we were to conjugate manger in the perfect tense with the first person singular it would be: j'ai mangé