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 (82 votes)

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
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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