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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Submitted by BobMux on Sat, 12/06/2021 - 05:44

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

Submitted by majesticdagny on Tue, 30/03/2021 - 13:08

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!

Hi majesticdagny,

Yes! The passive is fine to describe this action/event.


The LearnEnglish Team

Submitted by wasan0909 on Mon, 22/03/2021 - 22:05

-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

Submitted by Leila77 on Sat, 06/03/2021 - 09:40

Hi, Is there any difference between "can't have" and "coudn't have"?
Hi, I really appreciate your quick response to my question. Thanks a lot.

Submitted by Via on Thu, 15/10/2020 - 06:24

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

Submitted by kyawkyawsoezhu on Tue, 08/09/2020 - 17:59

*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

Submitted by Jamalmoghni on Sun, 09/08/2020 - 04:17

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

Submitted by Raazg9894 on Tue, 28/07/2020 - 12:22

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

Submitted by Kirk on Mon, 22/02/2021 - 14:00

In reply to by Eman_Alhindal


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

Submitted by Inci Ozturk on Tue, 21/07/2020 - 05:18

"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

Submitted by ryankht on Mon, 20/07/2020 - 23:32

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

Submitted by Karan Narang on Thu, 09/07/2020 - 04:36

yesterday I may not have later in the own by father didn't tell me sort of about why could you have come late in the shop.

Submitted by vinod on Tue, 09/06/2020 - 19:54

Hi sir . I am vinod from india. My question is this - let's say mr David works at a place since last ten years ,which is very far away from his home and he has to every day reach his place of work at 7 am dot. When I meet this mr David for the first time he tells me that he has to reach his office at 7 am and he travels by train . So I ask him after hearing all this - mr David then you may have to wake up very early everyday day to catch an early train so that you can reach office on time? My question is that is this use of ' may have to ' right here ? I am taking a guess here that mr David has been waking up very early since last few years and he still perhaps has to wake up early. And my second question can I replace may with must or might , if yes then , what will the sentence means? Sorry very long question but I kindly request you to answer my query.

Hello Vinod,

Yes, it's possible to use may have in this way but only for something which is not part of a constant routine. It implies that you are speculating, and it's most often used with a verb which also indicates this:

I guess you may have to...

I suppose you may have to...

When we are talking about a situation which is constant - something normal such as an everyday routine rather than something Mr. David might do on a particular occasion - then just have to is used:

I guess you have to...

I suppose you have to...


You could replace may with might in the first instance without changing the meaning.

It is possible to say must have to. The meaning is something like must be obliged to and it is used when you are speculating about what is required of another person.



The LearnEnglish Team

Mr Peter sir I truly appreciate your such a quick response . I never expected some one sitting far away and who does not know me to give a lucid explanation but u r great. May god bless u and ur family! In future if I have more questions,can I ask you?

Hi vinod,

Thank you for saying so! We try to response as quickly as we can, though sometimes it takes some time. If you post a question in the comments section of any page it will be read by someone on our team and we will try to respond. It may be me or it may be one of my colleagues - there's no need to address the question to anyone in particular.



The LearnEnglish Team

Submitted by carmenwf.jung on Tue, 12/05/2020 - 12:53

Thanks. I am still confusing little bit about modal using past participle. - He must do it ( present) - He must have done it (past) To simplify , when we mention a deduction about something happened in the past, does it always come modal verb with past principle?

Submitted by Peter M. on Wed, 13/05/2020 - 06:55

In reply to by carmenwf.jung


Hello carmenwf.jung,

Yes, that's correct. When we are making deductions about the past we use perfect modal verbs:

must have + past participle (very sure something happened)

could/might/may have + past participle (uncertain whether something happened)

can't have + past participle (very sure something did not happen)



The LearnEnglish Team

Submitted by Rsb on Tue, 05/05/2020 - 10:48

Sir, "Suspect must have left the country" Here we feel sure about something in the past. If I say same sentence in the future time. Here we feel sure about something in the future. Suspect must have left the country by next morning. Are they correct can we use this structure must have +verb 3rd form in both past and future time??

Submitted by Peter M. on Wed, 06/05/2020 - 07:34

In reply to by Rsb


Hello Rsb,

I've answered this question on another page for you. We reply to questions as quickly as we can, though we are a small team here. If you post the same question multiple times then it only slows the process down.



The LearnEnglish Team

Submitted by Bharati on Sat, 22/02/2020 - 04:17

In the example, 'I might have left the air conditioning on' based on the context can represent present time meaning equivalent to a present perfect'I have left the air conditioning on'.Present perfects do have connections with the present time also.