An earthquake? That must have been terrifying!

We don't know for sure that Alex broke the coffee table. It might have been the dog.

How did she fail that exam? She can't have studied very much.

Who told the newspapers about the prime minister's plans? It must have been someone close to him.

The thief must have had a key. The door was locked and nothing was broken.

Oh, good! We've got milk. Mo must have bought some yesterday.

might have / may have

I think I might have left the air conditioning on. Please can you check?

Police think the suspect may have left the country using a fake passport.

May have is more formal than might have. Could have is also possible in this context but less common.

can't have / couldn't have

She can't have driven there. Her car keys are still here.

I thought I saw Adnan this morning but it couldn't have been him – he's in Greece this week.