Je n'ai pas vraiment d'idées à part :

You must look after your little brother / You must have good marks
You can't surf everyday on the net.

Désolé pour le peu d'imagination ;p
Meilleure réponse !
I must answer calls on my cell phone.
I musn't throw my phone on the wall.
I can play with it.
I can't eat my cell phone.

En espérant t'avoir aidé.
We must obey.
We can not vote for President until the age of 18.
We must work to meet school teachers and classmates.
We must respect the opinions of others and to accept different choices of his own. J'espere que sa t'aidera.