Bonjour
Elles se sont lavé les mains.
<em>EXPLANATION</em>
pronominal verbs
"se laver" is a verb that exists in the form non pronominal "laver" but with the same meaning.
the rule is the same that with the auxiliary "avoir". It agrees with the direct object <em>ONLY IF </em>this one is before the verb =>
elle s'est lavée. => <em>elle a lavé quoi => se (herself) agrees with the pronoun direct object before the verb "se"</em>
elle s'est lavé les mains <em>(direct object after the verb . No agreement=</em>
les mains qu'elle s'est lavées <em>(direct object before the verb : agreement with the D.Object</em>
<em>other explanation</em>
<em>ils se sont lavé les mains => ils </em><em>ont </em><em>lavé quoi ? les mains.</em>
<em>to ask the question, you use "avoir"...</em>
<em>so the rule with avoir is the right one.</em>
<em>other examples</em>
<em>ils se sont écrit des lettres // les lettres qu'ils se sont écrit</em><em>es</em>
<em>ils ont écrit quoi ? des lettres</em>
<em> ils se sont écrit. ==> ils ont écrit </em><em>à qui </em><em>= "se" is an indirect object. No agreement</em>
<em>-</em>
<em>I know it's quite difficult ☺; lots of French people do the mistake ....</em>
courage !!! hope this helps a little ☺☺☺
<em> </em>
