[Originally posted on 08/12/12 on the Duolingo French for English speakers forum by olimo]
You may have noticed that past participles used with “être” have to agree with the subject.
For example:
Je suis parti – a man or a boy is speaking
Je suis partie – a woman or a girl is speaking
Nous sommes partis – some men or men and women are speaking
Nous sommes parties – some women are speaking
Unlike this, the participles used with “avoir” don’t have to agree with the subject: Il a écrit une lettre, Nous avons écrit une lettre, etc.
But here comes a curious thing. If the direct object (like “une lettre” in the previous example) is substituted with a pronoun (la), it has to go before the past participle. And in that case, the participle has to agree with the direct object.
Examples:
J’ai écrit une lettre. Je l’ai écrite. (“Une lettre” is feminine → “la” (though contracted to “l’ ”) → écrite.)
J’ai vu mes parents. Je les ai vus. (“Les parents” is plural → les → vus.)
J’ai lavé les chaussettes. Je les ai lavées. (“Les chaussettes” is feminine plural → les → “e” for feminine and “s” for plural → lavees.)
Il m’a vue – I am a girl, so “me” (m’) here refers to the feminine direct object, thus it is “vue”, not “vu”. Similar: Il nous a vus – vus for masculine or mixed plural, vues for feminine plural.
Here is an example from a Duolingo lesson: Cette femme, je l’ai connue pendant deux ans. (“Connue” for “la” for “femme”.)
This is not only with pronouns, but with nouns as well.
La viande qu’il a cuisinée est dans le four. – “La viande” is feminine and comes before the participle, thus it is “cuisinée”.
To put it short:
The past participle used with “être” has to agree with the subject.
The past participle used with “avoir” and coming after the direct object has to agree with the direct object.
Duolinguists 