French Verbs with Two Auxiliaries
A small number of verbs in French can take either avoir or être as the auxiliary depending on whether the verb is being used transitively or intransitively.
The famous mnemonic diagram La maison d'être can be misleading as it usually shows someone going up and down stairs with monter and descendre but avoir is the auxiliary if the verb is followed by a noun.
J'ai monté les escaliers. - I went up the stairs.
It's only when there's no direct object that the auxiliary verb can be être:
Je suis monté avant lui. - I went up before him.
Être is also the auxiliary when monter and descendre are used to mean getting into and out of vehicles or spaces:
Je suis monté dans le grenier. (I went into the attic).
The following verbs can take either avoir or être:
|to go down / to get off
|to go in / to come in
|to go up / to get on
|to pass again
|to go back
|to go out