Modals – deductions about the past

Do you know how to use modal verbs to show how certain you are about past events?

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

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Language level

B2 English level (upper intermediate)

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!

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

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

hi i need help with this senario (“One day your colleague doesn’t arrive at work. This is unusual because he has never been sick in the
past. When you ask your boss about him, she tells you that he contacted the company to say that he was
sick. He returns to work two weeks later. He has lost a lot of weight but looks very healthy and has a good
suntan. You also notice that he is very calm when he works and does not seem to get as frustrated as he
did in the past with you or his other colleagues.) and this question about it Why did your colleague take time off work?
How can you explain his physical condition?
How can you explain his new calm manner?
A few days later you talk to your colleague. Here are some of things he told you:
He was feeling burned out and needed a break from work.
He went on a 10-day meditation vacation.
He now meditates regularly and follows a vegan diet.

Hi there, I would appreciate your help. The sentence: "The examples that did not fit the scheme or may have been described more accurately were processed additionally." Is "may have been" used correctly here? Thank you!

Hello Robi Pokrajac,

I think you probably need 'could have been', 'should have been' or 'needed to be' here, but it's really not possible to be sure without knowing the context and what you are trying to say.



The LearnEnglish Team

Thank you very much, Peter.
Well, my intention was to say something like that there were those additional examples that did not fit the original scheme and that there was a possibility of explaining them more accurately. A paraphrase would be "They were possible to be explained more accurately"
So I used may have been to indicate possibility
What do you think?

Hello again.

In that case I think 'could have been' is the best option. May have been would carry a sense that something was possible but unknown. Could have been can have this meaning too, but can also mean that there was the possibility of doing something differently. For example:

She could/may/might have missed the train. [this is a possibility; I don't know if it is true]

She could have worked harder. [the possibility was there but was not taken]

Your sentence seems closer to the second:

The examples that did not fit the scheme or could have been described more accurately were processed additionally.



The LearnEnglish Team

Hi Muhammadqodir,

"Needn't have" means that it was not necessary to do something (e.g. "We needn't have taken heavy jackets. It was a hot day."). It may imply it was a waste of time or resources.

"Shouldn't have" means that it was a mistake to do something, i.e. it's a criticism (e.g. "We shouldn't have taken heavy jackets. They were too heavy to carry.")

I hope that helps.


The LearnEnglish Team

hello the learnenghlish team. Can I express deduction by modals and to be used to (get used to) at the same time. e.g. Can I use that "My mom may have been used to getting early. She milked cow every morning"
Or is it wrong?
Thank you in advance

Hello Anstasia,

Yes, it's possible to do that. In this case, I might suggest using 'must' instead of 'may' ('My mom must have been used to getting up early'), but it's perfectly fine to say 'My mom may have been used to getting up early'. The one with 'may' expresses more uncertainty and the one with 'must' expresses more certainty -- as if it's what makes most sense.

All the best,
The LearnEnglish Team

Hello. Could you please help me? Which one is correct or both? Is there a difference?
A: Last term, I took four exams and passed them all!
B: You (couldn't - can't) have been disappointed.
Thank you.

Hello everyone,

Could you please tell me if this sentence is correct and how to use CAN after IF clause while using THIRD conditional:

I wouldn't have used the car park if I could park on the street

Thank you!

Hello Natasa Tanasa,

This is an example of an unreal past conditional in which both the condition (the if-clause) and the result (the result clause) are describing imaginary events in the past. The best option is 'had been able to':

I wouldn't have used the car park if I had been able to park on the street.



The LearnEnglish Team

Hello The LearnEnglish team, I would like to ask whether " must have, may have, could have+ past participle" can be used to guess events or situations with present result as we use present perfect tenses. In other words, do the structures above only refer to the past or something with present result?

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:

There is blood on the floor so she must have had an accident.

present evidence: blood; past action: having an accident



The LearnEnglish Team

I would like to know whether we can use the passive voice in this kind context. Would it be correct when I say "The building must have been built by the ruling coalition." Thank you in advance for the answer!
-she must've been studying hard the exam isn't easy -they couldn't make it to dinner, they might've been sick -he can have left already please give me some feedback are the sentences right?

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.



The LearnEnglish Team

Dear team, Does couldn't/can't have consider as negative sentences? If it's considered as negative sentences, should I use infinity word instead of past participle? Thanks a lot
*He couldn’t have known it when I saw him. I’m sure he would have told me.* Those sentences are from Grammar Test 2. I want to know about the second part of the sentences. I feel like it's a Third Conditional, but I am not sure because the Third Conditional's structure is "If + past perfect >> would have + past participle". Please help me.

Hi kyawkyawsoezhu,

Yes, it is part of a third conditional structure! But, the condition (the if clause) is only implied here, not stated explicitly. In full, it would be: I'm sure he would have told me if he'd known. But the first sentence implies the if he'd known part, so the speaker omitted it.

Does that make sense?

Best wishes,


The LearnEnglish Team

Hello English Team In making gusses about the past I can't understand when I use ( have been ) and only have without been I really get confused , please can you explain it to me with some examples ?

Hello Jamalmoghni,

There are many different forms which use have and have been, both as main verbs and as parts of other verbs (perfect forms, passive forms etc). Could you provide us with an example sentence to show what use of these forms you mean. Then we'll be sure we're giving you the right information.



The LearnEnglish Team

Hello English team, I am Raaz Gupta from India. My question is regarding the use of 'could have'. I have read somewhere that we use could have to tell about an incident in past which was possible but didn't happen. For example, He slipped on the banana, he could have hurt himself. Another one is- He could have missed the train, if hadn't reached on time. But what I see here is something different what I have read. Please shed some light on the use of 'could have'. Looking forward to your reply. Regards Raaz Gupta

Hello Raaz Gupta,

The modal verb 'could have' has many different uses. One of these is to express deductions about the past, and this is what is described on this page.

You can see other uses of 'could' (including 'could have') -- for example, to express possibility, as in your first example sentence -- explained on our 'can' and 'could' grammar page. I'd also recommend having a look at the Modals with 'have' page as well.

Please note that we don't generally comment on other grammars, but if you see something in the one you are familiar with that seems to differ from what you read here, you are welcome to ask us a specific question about it.

All the best,


The LearnEnglish Team

Hi there, Thank you so much for your support. (((That’s the most uninspiring documentary I have ever watched.))) I'm finding it difficult to understand the form of this sentence? I want to explain it for my paper. Your help would be appreciated. Thank you

Hello Eman_Alhindal,

This is a sentence with a superlative form ('the most uninspiring') and a reduced relative clause. If we add the relative pronoun back to the sentence, it is: '... documentary that I have ever watched'.

Does that help? Feel free to ask us a specific question if it's still unclear.

All the best,


The LearnEnglish Team

Thank you. "You can't be hungry" How is it different from "You're not hungry" "I'm attending the event next Friday" How is it different from "I'm going to attend the event next Friday" Appreciate you feedback.

Hello Eman_Alhindal,

If I say 'You can't be hungry' to you, it means that I think it's not possible that you are hungry. Perhaps I've just seen you eat a lot of food, for example, and so now it's hard for me to believe that you are still hungry.

'You're not hungry' is less specific and so it could be used in many situations. For example, maybe I believe you're not hungry because I just saw you eat, or maybe I'm encouraging you not to eat when you appear tempted to have a piece of cake. The meaning here is much more dependent on the context than the first one.

As for the last two sentences, in many cases they would mean the same thing. The first one could suggest you've already made arrangements to go -- i.e. that you already have a ticket for the event -- than the second one, which could indicate an intention to go more than having taken action to go (i.e. you might not have a ticket). But in ordinary speaking, most of us are not so precise, and so you could often say one or the other and mean the exact same thing.

All the best,


The LearnEnglish Team

"I can't have left my wallet in the restaurant. I paid for the taxi home afterwards." I did not understand this answer. Is not he aware of his wallet whether he forgot or not? Why don't we say "I must have left my wallet in the restaurant. I paid for the taxi home afterwards."? I understand that he forgot his wallet in the restaurant. Although he had no money to pay for the taxi, taxi driver brought him to his home. Then he got off the taxi, brought back money to the taxi driver from home and then he was able to make the payment finally.

Hello Inci Ozturk,

Can't have is used to express something that the speaker thinks was not possible. Thus, the speaker is saying this:

It's not possible that I left my wallet in the restaurant. I paid for the taxi home afterwards.

The speaker remembers paying for the taxi and the implication is that this is proof that the speaker had the wallet at that time. Therefore it is not possible that it was left it in the restaurant.

Must have would have the opposite meaning.



The LearnEnglish Team

Hello, English team, Some questions want to clarly, thank you 1. can I use mustnt have, mightnt have, could have to show the past indication 2. What is the difference on may, might and can, could in expreesing the past indication

Hello ryankht,

Perfect modals like mustn't have do have a past meaning, but they are not past forms of the modal verbs. For example:

He must leave at 6.00. [present/future obligation]

He had to leave at 6.00. [past obligation]

He must have left at 6.00. [deduction about the past]


With modal verbs, the context is crucial. I think for your second question you really need to provide concrete examples of what you have in mind so that we can be sure we understand properly.



The LearnEnglish Team

Thanks Peter, really appreciate your help, In the page, it teach us we can use must have + pp ->> show sth very possible to happen might have + pp ->> show sth possible to happen couldnt have + pp ->> show sth impossible to happen I want to know if we can use mustnt have +pp, could have +pp, mightnt have + pp to show the past indiciations, because i do not see those example in the page, not sure if i can use them For the second question, could you show me some examples of the difference in may might and could can in the past indiciations. I have no idea on it, i think they are the same when we use them to show past indiciations