Tricky past-participle agreement

Past-participle agreement is where even native speakers hesitate. The core rule with avoir: the participle agrees with a direct object placed before it. The hard cases follow from that.

Pronominal verbs

Ask what the reflexive pronoun is. If it's the direct object, agree; if something else is the direct object, or se is indirect, don't.

• Elles se sont lavées.They washed (themselves). (se
• Elles se sont lavé les mains.They washed their hands. (les mains
• Les mains qu'elles se sont lavées.The hands they washed. (que
• Ils se sont téléphoné.They phoned each other. (téléphoner à: se is indirect)

Verbs like se parler, se sourire, se succéder, se plaire, se mentir never agree.

fait + infinitive: never agrees

• La maison qu'il a fait construire.The house he had built.

Other participles + infinitive

Agree if the preceding object does the action of the infinitive:

• Les chanteuses que j'ai entendues chanter. (the singers sing)
• Les chansons que j'ai entendu chanter. (the songs don't sing)

Since the 1990 spelling reform, laissé + infinitive is invariable like fait.

coûter, valoir, peser, courir, vivre

Agreement only in the figurative sense, where they take a real direct object:

• Les efforts que ce travail m'a coûtés.The efforts this work cost me.
• Les mille euros que ce sac a coûté.The thousand euros this bag cost. (a measure: no agreement)

Related: Passé composé with être, Pronominal verbs.