Hi Damien,

you are correct in that the past participle does not match the COD in the case of avoir

BUT (and there always is a but!)

except for the case when the COD precedes the particple, in which case it IS matched. Let's look at your example:

Nous les avons préparés. --"les" is the COD and precedes the participle ("préparés"). Since "les" is plural (and apparently not all female), the participle needs to take this into account --> "preparés".

-- Chris (not a anative speaker).

nous les avons préparés. Why not préparé? i thought past participle did not change to match subject when avoir is auxiliary.

thanks Chris, very interesting. By the way, what does COD stand for? i would have called "les" a direct object pronoun. 

COD = Complément d'objet direct.

Yes, "direct object pronoun" is exactly what COD means.

-- Chris.