Réponse :
Teacher : No, Andrew, you mustn't jump around in the classroom! You must sit down. And you mustn't box your neighbours! You mustn't eat during my lesson and mustn't drop your litter on the floor! Pupils must be quiet!
Must (devoir - c'est un ordre) : You must do your homework! = Tu dois faire tes devoirs!
Mustn't (le contraire de devoir - c'est une interdiction) : You mustn't fight with your sister = Tu ne dois pas te bagarrer avec ta soeur
Voilà j'espère que ça t'a aidé!