With the auxiliary "être", the past participle always agrees in gender and number with the subject of the verb. This is the simplest of the past participle rules.
With "être", the participle always agrees with the subject.
Examples
Elles sont parties tôt ce matin.
Key word(s): parties
The subject "elles" is feminine plural, so "parties".
Il est arrivé hier soir.
Key word(s): arrivé
The subject "il" is masculine singular.
Les feuilles sont tombées.
Key word(s): tombées
The subject "feuilles" is feminine plural.
Nous sommes allés au marché.
Key word(s): allés
The subject "nous" is masculine plural (or a mixed group).
Marie est venue avec sa soeur.
Key word(s): venue
The subject "Marie" is feminine singular.
Common mistakes
Forgetting the feminine agreement ("elle est arrivé" instead of "arrivée")
Confusing this with the auxiliary "avoir", which follows a different rule
Forgetting the plural -s ("ils sont allé" instead of "allés")
Test yourself
Les enfants sont ___ à l'école.
"Enfants" is masculine plural (or a mixed group), so "allés".
Ma soeur est ___ en vacances.
"Soeur" is feminine singular, so "partie".
Les portes sont ___.
"Portes" is feminine plural, so "ouvertes".
Tips to remember
Always identify the subject first.
With "être", treat the agreement like an adjective: "elle est grande" works the same way as "elle est partie".
Verbs of movement often take "être": aller, venir, partir, arriver, tomber, and others.
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