Expresando obligación y prohibición en inglés/ Expressing obligation and prohibition in English: 'must' and 'mustn't'

Must is a word that is used to talk about an obligation, something you have to do, like following a law or a rule.
Mustn't is a word that is used to talk about a prohibition, about something that is forbidden.

Please remember that mustn't is NOT the negative form of must.

STRUCTURE	must / mustn't + verb
Positive sentences	I must do my homework. She must wear a uniform.
Negative sentences	We mustn't talk in here! You mustn't steal.
Interrogative sentences	Must I go out now? Mummy, must we brush our teeth after dinner?
All pronouns take must/mustn't	I You She He It We They	must / mustn't	have a visa. buy a ticket. smoke in here. have long hair.

Do some exercises and improve your knowledge clicking here:

Fill the blans with must or mustn't