MONTER, DESCENDRE, SORTIR

Monter—"to climb, climb in", descendre—"to get off, climb down," and sortir—"to leave" generally take être in the Passé Composé.

These three verbs can have other meanings though, and when they do, they require avoir in the Passé Composé.

monter—"to carry up, bring up"
 

Je monte l'armoire.

I bring up the wardrobe.

J'ai monté l'armoire.

I carried the wardrobe up.

descendre—"to carry down, bring down"
 

Je descends l'armoire.

I bring the wardrobe down.

J'ai descendu l'armoire

brought the wardrobe down.

sortir—"to take out"
 

Je sors la clé de ma poche.

I take the key out of my pocket.

J'ai sorti la voiture du garage.

I took the car out of the garage.

Je l'ai sortie. (la voiture)*

I took it out.

*A preceding direct object requires agreement of the Past Participle with the direct object.