Modals: deductions about the past

Modals: deductions about the past

Do you know how to use modal verbs to show how certain you are about past events? Test what you know with interactive exercises and read the explanation to help you.

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

Modals – deduction (past): 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

Modals – deduction (past): Grammar test 2

Language level

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Thanks a lot! I found this sentence in a grammar exercise book and had to open the brackets with the correct form but it really seemed a bit odd with the word "guess". Thank you for your clear explanation, it helped me a lot! Thanks for your time and attention!


Modal verbs like these have several different uses:


ability - Can you play the piano?

theoretical possibility - Anyone can make a mistake. 

permission - Can I leave early, please?

requests - Can you give me a hand?

offers - Can I help you?



possibility - We may go to France next year.

expressing hopes - May you both be very happy.

permission - You may go when you've finished.

concession - They may live next door but we hardly ever see them.


I hope that helps to clarify it for you.



The LearnEnglish Team

Submitted by riverolorena67 on Fri, 05/08/2022 - 19:17


Is it grammatically correct to write this sentence when you are using a modal verb for deduction in the past?
A friend lent him a gun.He was worried somebody might have killed him or his dsughters

Hi riverolorena67,

Yes! It's a correct sentence (but needs a spelling correction --> daughters). 

Actually, here, "might have" shows possibility in the past, not deduction. Deduction about the past means reaching a conclusion about what has already happened, based on evidence or reasoning - but the action ('killed') hadn't happened yet, at the time that he was worried. Instead here, it's the regular meaning of "might" (i.e. possibility), but about the past.

I hope that helps.


The LearnEnglish Team

Submitted by Kabir Sharma on Fri, 05/08/2022 - 10:04


Hello to the British Council team.
Here's the example:
The tasks that could have been done correctly were done correctly.
It means that those tasks which were possible to be done correctly in the past were indeed done correctly.
It confuses me since could have been implies that the action in question did NOT take place in the past, while here it obviuosly did, as the tasks were done correctly. "Were done correctly" can also be substituted by something like "were taken care of", which would still implicitly tell us that the action took place.

Thanks. Bye

Hi Kabir Sharma,

I see what you mean, and I can see why confusion is possible. But for me the intended meaning of the sentence is clarified by the "were done correctly" part, as you pointed out. 

To avoid confusion, we could simply use "could" --> "The tasks that could be done correctly were done correctly." 

Best wishes,


The LearnEnglish Team