"You must behave."
Translation:Jullie moeten je gedragen.
Why can't you say "Jij moet jezelf gedragen"? It said it had to be "je" instead of "jezelf"
Zich is the reflexive pronoun for the third person singular and the formal u (http://www.dutchgrammar.com/en/index.php?n=Pronouns.Rf01), for the informal je/jij the reflexive pronoun is je.