Look at these examples to see how must, might, may, could, can't and couldn't are used in the past.
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.
Try this exercise to test your grammar.
- Grammar test 1
Read the explanation to learn more.
Grammar explanation
We can use modal verbs for deduction – guessing if something is true using the available information. The modal verb we choose shows how certain we are about the possibility. This page focuses on making deductions about the past.
must have
We use must have + past participle when we feel sure about what happened.
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
We can use might have or may have + past participle when we think it's possible that something happened.
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
We use can't have and couldn't have + past participle when we think it's not possible that something happened.
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.
Do this exercise to test your grammar again.
- Grammar test 2
Hello BobMux,
Perfect forms relate a later state or event to an earlier state or event, and the perfect modal forms are no different.
These forms are used for deductive reasoning, relating present evidence or knowledge to a past situation or action:
Peter
The LearnEnglish Team
Hi majesticdagny,
Yes! The passive is fine to describe this action/event.
Jonathan
The LearnEnglish Team
Hello wasan0909,
Aside from punctuation, the first two sentences are correct, though we would only write contractions like might've in a very informal context; generally we only contract have when used with pronouns (I, you, we etc).
The third sentence is not correct. We don't use can have for speculation. You could use might have, may have or could have instead.
Peter
The LearnEnglish Team
Hello Leila77,
In the context of deduction about the past as described on this page, there is no difference.
For more information about can and could, including can have and could have, you can visit this page:
https://learnenglish.britishcouncil.org/english-grammar-reference/can-and-could
Peter
The LearnEnglish Team