Speculating about the past
Strong belief: You have just run a marathon. You must be tired.
Jenny must be very pleased with her test result. An A+ on her English test!
That can't be right. I ordered a yellow sweater, not a green one!
That can't be Peter. He's in the US at the moment.
Strong belief about the past:
Carl ran a marathon yesterday. He must have been very tired afterwards.
Jenny must have been very pleased with the mark for her English test last week.
They can't have sent the right sweater. I ordered a yellow one.
You can't have seen Peter at the party last night. He's in the US right now.