Auxiliary selection: avoir vs être
Summary
Compound tenses use avoir as the auxiliary for most verbs, but être for all pronominal verbs and a small set of intransitive verbs of motion or change of state.
Explanation
French compound tenses (passé composé, plus-que-parfait, futur antérieur, conditionnel passé, etc.) are built from an auxiliary verb plus the past participle. The first decision is always which auxiliary to use, and the default is avoir: the great majority of verbs take it, including all transitive verbs (j'ai mangé une pomme, elle a vu le film, nous avons fini). If in doubt, avoir is the statistically safe choice.
Être is used in two well-defined cases. First, with every pronominal (reflexive) verb without exception: se lever, se laver, se souvenir all take être (je me suis levé, elle s'est lavée). Second, with a short, closed list of intransitive verbs of motion or change of state, traditionally remembered by the mnemonic DR & MRS VANDERTRAMP: devenir, revenir, monter, rester, sortir, venir, aller, naître, descendre, entrer, rentrer, tomber, retourner, arriver, mourir, partir — plus passer in many uses.
Choosing être has a grammatical consequence: the past participle agrees in gender and number with the subject. So elle est partie, ils sont partis, elles sont venues. With avoir there is normally no agreement with the subject (elle a mangé, not *mangée), though the participle agrees with a preceding direct object — a separate rule. This subject agreement under être is one of the most visible markers of correct written French.
Two pitfalls dominate. The first is using avoir with a pronominal verb (*elle s'a levée instead of elle s'est levée): pronominal verbs always take être. The second is that several DR & MRS VANDERTRAMP verbs switch to avoir when they become transitive — that is, when they take a direct object: il est monté (he went up) but il a monté les valises (he carried the suitcases up); elle est sortie (she went out) but elle a sorti son passeport (she took out her passport). The auxiliary follows the construction, not just the verb.
Rule of thumb: use être for all reflexive verbs and for the DR & MRS VANDERTRAMP motion/state verbs, make the participle agree with the subject when you do, and switch back to avoir the moment one of those verbs takes a direct object; otherwise use avoir.
Examples
J'ai mangé une pomme, mais elle est partie tôt. — I ate an apple, but she left early. (manger takes avoir; partir takes être with subject agreement (partie).)
Nous nous sommes levés à six heures. — We got up at six o'clock. (Reflexive verbs always use être.)
Ils sont arrivés en retard. — They arrived late. (arriver is on the être list; participle agrees (arrivés).)
Il est monté au troisième étage. — He went up to the third floor. (Intransitive monter takes être.)
Il a monté les valises au grenier. — He carried the suitcases up to the attic. (With a direct object, monter switches to avoir.)
Common mistakes
Elle s'a levée à six heures.
Elle s'est levée à six heures.
All pronominal (reflexive) verbs form their compound tenses with être, never avoir: elle s'est levée, nous nous sommes lavés. Using avoir ("elle s'a levée") is wrong. Because the auxiliary is être here, remember that the participle also tends to agree (s'est levée), unlike with avoir.
Prerequisites
Related rules
auxiliariespasse-composecompound-tensesparticiple-agreement