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.
  
seatbelt.jpg

2. To emphasize the necessity of something.
 
You must study hard before the test.
You must drive carefully.
  
3. Deduction / Certainty
  
Dinosaurs were huge, they must have eaten a lot.
  
dinosaurs-wallpaper-4(1).jpg
  
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.
  
скачанные файлы.jpg
  
5. A strong recommendation.
  
Something that is highly recommended (stronger than "should")
 
The ice cream is delicious. You must try some.
  
ice-cream.jpg

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.
  
texting_2161419b.jpg
Quellen:
http://www.grammar.cl/english/must.htm
tx.english-ch.com
www.atlanteanconspiracy.com
airambulance.co.uk
www.thegoodshoppingguide.com
www.telegraph.co.uk