A small, fixed group of verbs, mostly about movement or a change of state, use être instead of avoir as their passé composé auxiliary. There aren't many of them, and they're worth memorizing as a set.
1. The être verbs: DR & MRS VANDERTRAMP
A classic memory trick, each letter is the first letter of one of these verbs.
| Devenir, Revenir | to become, to come back |
| Monter, Rester | to go up, to stay |
| Sortir, Venir | to go out, to come |
| Aller, Naître | to go, to be born |
| Descendre, Entrer | to go down, to enter |
| Rentrer, Tomber | to go home, to fall |
| Retourner, Arriver | to return, to arrive |
| Mourir, Partir | to die, to leave |
2. The participle agrees with the subject
Because the auxiliary is être, the past participle behaves like an adjective, matching the subject's gender and number.
| Il est allé au marché. | He went to the market. |
| Elle est allée au marché. | She went to the market. (-e added) |
| Ils sont partis tôt. | They left early. (-s added) |
| Elles sont arrivées en retard. | They arrived late. (-es added) |
3. Watch out: this is a closed list
Not every verb of movement: common verbs like marcher (to walk), voyager (to travel), or courir (to run) still use avoir, they're not on the list. J'ai marché, not "je suis marché."
4. Conjugation: present tense (model: aller)
| je suis allé(e) | I went |
| tu es allé(e) | you went |
| il est allé / elle est allée | he / she went |
| nous sommes allé(e)s | we went |
| vous êtes allé(e)(s) | you went |
| ils sont allés / elles sont allées | they went |