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

Average: 4 (84 votes)

Hello wilson3827,

The sentence is fine. The first part describes an imagined past and the second describes an imagined result. In conditional constructions we generally do not use a modal verbs in the if-clause (If the referee had... then... would have...), but this is not a conditional construction - perhaps this is the source of your uncertainty.



The LearnEnglish Team

Submitted by Marco Gaiotto on Fri, 18/11/2022 - 22:24


Few people went to the lecture about animal sounds: she could/must have been very disappointed.

Hello! I'd like to ask you if both options are acceptable here. It's a single sentence taken from an exercise, no further context is given.
Must have been refers to the idea of "certainty", while "could have been" conveys the concept that "she was probably disappointed, but maybe not".
What would you suggest? Thanks a lot beforehand!

Hi Marco Gaiotto,

"Could have been" is certainly grammatically possible and meaningful here, and if this was said in a real life conversation it would be acceptable. But "must" seems the most obvious answer and that would be my first choice, in a grammatical practice exercise. 

I hope that helps.


The LearnEnglish Team

Submitted by Liam_Kurt on Fri, 18/11/2022 - 09:43


Hello, I need some help on this question. Is adding has + present perfect to a modal verb the same as a perfect infinitive? So is "you should have done the same" as "you are supposed to have done"?

Hello Liam_Kurt,

It's not exactly a perfect infinitive, but the meaning is similar. You can find an explanation of this on our Modals with 'have' page. Please have a look, and then if you have any additional questions, don't hesitate to ask!

All the best,
The LearnEnglish Team

Submitted by Lana.Curi.Kolesnyk on Sun, 25/09/2022 - 19:35


Hello! I'm Lana. I have a question about the modal verb must the answer to which I can't find in any grammar book. Hope, you will be able to help me. Is it correct to say "I guessed he must be waiting for me" in the past (if must here is in the function of probability implying assurance)? Or do I have to use "must have been waiting" as it's a sequence of tenses?
Thanks for your answer!

Hello Lana,

As you suspect, 'I guessed he must be waiting' doesn't work because of the conflicting past and present time references.

'I guessed he must have been waiting' is correct in terms of the sequence of tenses but is still a little odd. Unless I've misunderstood the intention or context, 'he must have been waiting' means something like 'I had good reason to believe he was waiting'. If that's correct, then saying 'I guessed' before it is a little confusing because it implies I supposed he was waiting but didn't have very solid reasons for believing this.

Our Modals – deductions about the past page describes this grammar, though it sounds to me as if you're already familiar with it.

Hope this helps.

All the best,
The LearnEnglish Team

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