Uses of "must" (negative: must not, mustn't)

1. To show that you have to do something, for example because it is very important or because it is a rule
   "You must stop the car when the traffic light turns red."
   "You must pay your taxes."
   "She must stop drinking if she wants to keep her job."
   "I must go now, otherwise I will miss my train."
   
2. "Must not" (or "mustn't") is used to show you are not allowed to do something
   "You mustn't steal."
   "He mustn't talk to his parents like that."
   "The fruit of this bush must not be eaten because it is toxic."
   
3. To show that something is very logical or very likely to be true
   "He left at noon, so he must be there already."
   "She is not stupid, so she must have known what she was doing!"
   "They must be really rich to live in such a house."