Teorija

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 - sure that something is true (Certainty).
  
We use this when we don't know but we are certain that it is true (based on evidence).
 
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 the lottery with the new house and car he has just bought.
  
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5. A strong recommendation.
  
Something that is highly recommended (stronger than using 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.
  
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Atsauce:
http://www.grammar.cl/english/must.htm
tx.english-ch.com
www.atlanteanconspiracy.com
airambulance.co.uk
www.thegoodshoppingguide.com
www.telegraph.co.uk