When do you need to agree the past participle when using the passé composé?

Participles are tricky little things, but when it comes to agreeing them, all you've got to do is learn the rules – there aren't any pesky exceptions!

Participles agree in gender and number. To form the plural, add an -s; to form the feminine, add an -e. (To form the pl. fem., add an -es.)

1. Agrees with subject – after auxiliary être

• When the participle is used with the auxiliary être, the participle automatically agrees with the subject.

Thus: She fell out of the tree – Elle est tombée de l'arbre or We arrived on time – Nous sommes arrivés (ou arrivées) à l'heure. 

2. Agrees with object – when object precedes participle

• This is where it gets complicated. Basically, in phrases such as Il l'a choisi, l' is a pronoun which replaces the object and the object therefore comes before the participle in the sentence. When this is the case, the participle agrees with it.

Thus: She saved them – Elle les a sauvés (ou sauvées) or We ate them (say, the peaches) – Nous les avons mangées (les pêches).

I hope this clears some things up – now get practising!