Sometimes, the past participle remains invariable (masculine and singular), sometimes it agrees with another word in the sentences. Look at the same verb MANGER in these 2 examples:
J’ai mangé les pommes AND Je les ai mangées
The past participle may or may not agree in gender (add an E if feminine) and number (add an S if plural).
- Verbs that take AVOIR agree – or not – with the object
- Verbs that take ÊTRE agree – or not – with the object pronoun that reflects the subject
Here is how it goes:
Verbs that take AVOIR (goes with the object)
1. Verbs that take AVOIR do not agree if the object is after the verb.
J’ai mangé les pommes. --> no agreement with object because les pommes is after the verb
Verbs that take ÊTRE (goes with the object pronoun)
2. Intransitive verbs that take ÊTRE agree with the pronoun.
Elles se sont lavées. --> agreement with pronoun se=elles because se laver doesn’t have another object. The action is done to self
Related: le passé composé