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
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
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,
Kirk
The LearnEnglish Team
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,
Kirk
The LearnEnglish Team
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,
Kirk
The LearnEnglish Team
Hello Inci Ozturk,
Can't have is used to express something that the speaker thinks was not possible. Thus, the speaker is saying this:
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.
Peter
The LearnEnglish Team
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:
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.
Peter
The LearnEnglish Team