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)

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
Profile picture for user Kirk Moore

Submitted by Kirk Moore 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