How do I know when to use avoir or être as the auxiliary verb in the perfect tense?

Avoir is most commonly used in this case. So, if in doubt, it's safer to use avoir. However there are certain verbs that use être when forming the perfect tense. These are Reflexive Verbs and DR MRS VANDERTRAMP verbs. Reflexive verbs are ones that have se at the beginning of their infinitives. These include verbs like se détendre (to relax) and se laver (to wash yourself). When using the different pronouns the se changes to: Je me, Tu te, Il/Elle/On se, Nous nous, Vous vous, Ils/Elles se.Se détendre: Je me suis détendu(e)DR MRS VANDERTRAMP verbs are the following: Devenir,Revenir,Monter,Rentrer,Sortir,Venir,Aller,Naître,Descendre,Entrer,Retourner,Tomber,Rester,Arriver,Mourir,PartirAller: Je suis allé(e)Remember that when using être, the past participle needs to agree with the person.