Theorie:
Study the information about ‘must’ .
Must
1. To express obligation or duty.
This also refers to laws and regulations.
You must wear a seatbelt at all times.
2. To emphasize the necessity of something.
You must study hard before the test.
You must drive carefully.
You must drive carefully.
3. Deduction / Certainty
Dinosaurs were huge, they must have eaten a lot.
4. Expresses positive logical assumptions (Must + have + past participle).
He must have won in the lottery with the new house and car he has just bought.
5. A strong recommendation.
Something that is highly recommended (stronger than "should")
The ice cream is delicious. You must try some.
Mustn't
The negative is mustn't which refers to prohibition (negative obligation)
Mustn't = Must not
You mustn't use your mobile phone while you are driving.
Quellen:
http://www.grammar.cl/english/must.htm