Sorry, accidentally pressed enter :D Anyhow, I was wondering about a particular sentence : "Ils se sont lavé les mains." Does "lavé" not agree with "les mains" because the COI is before the COD? I didn't find a sentence like this in the lesson so I figured I'd ask. Merci! :)

There is no agreement when something other than the person(s) is having the action of the verb done to them. So, Ils se sont lavés has an agreement because ‘they are washing themselves' (therefore, the reflexive pronoun ‘themselves’ se is the direct object which requires an agreement) but Ils se sont lavé les mains has no agreement because 'They're washing their hands' (the hands are now the direct object, ‘themselves’ se has become the indirect object pronoun and doesn’t require an agreement).