We can use musno say that we are sure about something (because it is logically necessary).

• If A is bigger than B, and В is bigger than C, then A must be bigger than С
  
• Mary keeps crying. She must have some problem.
  
• There's the doorbell. It must be Roger.
  
• 'I'm in love.' 'That must be nice.'
  

In questions and negatives, we use can and can't with this meaning, not must and mustn't.

• 'There's somebody at the door. Who can it be? 'It can't be the postman.
• It's only seven o'clock.' What do you think this letter can mean?

 
We use
 
must have + past participle    for deductions about the past
 
(can have in questions and negatives).

must/can/can't have + past participle

• 'We went to Rome last month.' 'That must have been nice.'
  
• I don't think he can have heard you. Call again.
  
• Where can John have put the matches? He can't have thrown them away.