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

Try another grammar lesson

Language level

B2 Upper intermediate

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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.

Peter

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.

Peter

The LearnEnglish Team

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?

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)

Peter

The LearnEnglish Team

It's really helpful.

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??

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.

Peter

The LearnEnglish Team

Apologize for it!

Modals: deductions about the past

Grammar explanation

must have

might have / may have

can't have / couldn't have

Language level

Online courses