Forming the perfect (or pluperfect) tense in French is sometimes made harder than necessary by what’s called a Preceding Direct Object (or PDO). The object of a sentence is whatever ‘suffers the action of the verb’, eg the nail in ‘he hit the nail on the head’. If the object is a pronoun and the perfect (or pluperfect) tense is being used, the French put it before the auxiliary rather than at the end of the sentence as in English, eg ‘il l‘a frappé’ or ‘he it has hit’!
You might think that’s bad enough, but the real problem is that the past participle has to agree in number and gender with the PDO, eg ‘il les a frappés‘ or ‘he’s hit them’.
When it comes to reflexive verbs, it gets even worse. By definition, every reflexive verb has a PDO, so that means you have to watch what you write when a female character is speaking, eg ‘Je me suis lavé’ for a boy, but ‘Je me suis lavée’ for a girl. Having said that, one of my male pupils once told me that the best way to get a good mark in his French prose was to tell a story in the first person and pretend to be a girl – that way, he’d get a tick every time he used a PDO!