Theory:

Must is a verb. It is used to indicate obligation, necessity, or an opinion about something that is logically very likely.
How is the verb "must" used in sentences?
1. To express obligation or duty.
 
This also refers to laws and regulations.
  • Example:
    You must wear a seatbelt at all times.
seatbelt.jpg
 

2. To emphasize the necessity of something.
    Example:
  • You must study hard before the test.
  • You must drive carefully.
  
3. Deduction - sure that something is true (Certainty).
 
We use this for situations where we are certain of something even though we lack a real time experience on that. The deduction is based on evidence.
  • Example:
    Dinosaurs were huge, they must have eaten a lot.
dinosaurs-wallpaper-4(1).jpg
 
 
4. Expresses positive logical assumptions (Must \(+\) have \(+\)  past participle).
  • Example:
    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")
  • Example:
    The ice cream is delicious. You must try some.
ice-cream.jpg
Mustn't/ Must not

The negative "mustn't"  refers to prohibition (negative obligation)  
  • Example:
    You mustn't use your mobile phone while you are driving.
texting_2161419b.jpg
Reference:
http://www.grammar.cl/english/must.htm
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