Rule: agreement between the SUBJECT and the PAST PARTICIPLE
Elle est tombée
“Elle” is feminine so we add an “e” at the end of “tombé”
Ils sont restés (they stayed)
“Ils” is masculine plural so we add an “s” at the end of “resté”
Je suis partie (I left) (Je = Charlotte)
“Je” is feminine so we add an “e”
It is that simple!
The rule is different with reflexive verbs, they behave like AVOIR!
Ask the questions WHAT and WHERE to know what is the direct object and where it is.
Rule: No Agreement.
Je me suis brossé les dents (I brushed my teeth), we could also say: j’ai brossé mes dents, that’s why the behaviour is the same.
What is the Direct Object? answer the question, what did I brush? my teeth, so the Direct object is “dents”
Where it is? After the verb être so no agreement! Brossé!
Rule: Agreement between the past participle and the direct object.
Je me les suis brossé (I brushed them), we could say: je les ai brossé. “Les” is for “dents”, so feminine plural!
Is that sentence written correctly?
What is the direct object here? les.
Where it is: before the verb être so there is an agreement between the direct object and the past participle Je me les suis brossées is the correct answer (and not as written above).