Study the examples below:

(1)

She lives in the mountains. The view must be amazing up there.

He hasn’t learned enough for this exam. He can’t know everything.

We use must to say we believe something is certain.

We use can’t to say we believe something is not possible.

 

(2)

I called her yesterday but she didn’t answer. She must have lost her phone.

Henry didn’t pass his exam. He can’t have learned enough.

We use must have (+ past participle) and can’t have (+ past participle) to talk about the past.