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Look at these examples to see how must, might, may, could and can't can be used.

That must be the main entrance. I can see people queuing to get in.
I've lost my keys. They might be at work or they could be in the car.
You can't be bored already! You've only been here five minutes.

Try this exercise to test your grammar.

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 present or future.

We use must when we feel sure that something is true or it's the only realistic possibility.

This must be her house. I can see her car in the garage.
He must live near here because he always walks to work.
Come inside and get warm. You must be freezing out there!

We use might, may or could to say that we think something is possible but we're not sure.

She's not here yet. She might be stuck in traffic.
He's not answering. He could be in class.
We regret to inform you that some services may be delayed due to the bad weather.

They all have the same meaning, but may is more formal than might and could.

We use can't when we feel sure that something is not possible.

It can't be far now. We've been driving for hours.
She can't know about the complaint. She's promoted him to team leader.
It can't be easy for him, looking after three kids on his own.

Note that these verbs, like all modal verbs, are followed by an infinitive without to.

Do this exercise to test your grammar again.

Language level

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Do you know how to use modal verbs to say how certain you are about a possibility? Test what you know with interactive exercises and read the explanation to help you.

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Wordwall - Crea mejores lecciones de forma más rápida

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MODALS OF DEDUCTION and SPECULATION Myri

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Cada oveja con su pareja - Toca en la respuesta correspondiente para eliminarla. Repite hasta que todas las respuestas se hayan ido.

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We use have to / must / should + infinitive to talk about obligation, things that are necessary to do, or to give advice about things that are a good idea to do.
Must and have to are both used for obligation and are often quite similar. They are both followed by the infinitive.

Are these exactly the same?

Well, almost. We often use must for more personal opinions about what it is necessary to do, and have to for what somebody in authority has said it is necessary to do.

I must remember to get a present for Daisy. (my opinion)
You have to look after their hair regularly. (dog experts say so)
Do you have to wear a tie for school? (asking about school rules)

Which verb do people use more?

Have to is more frequent in conversation; must is used more in formal writing, for example in written notices.
Passengers must fasten their seat-belts.
Have changes in the third person singular ( he/she/it has); but must doesn't change. It's a modal verb and modals don't change.

I think I've heard have got to. Is that correct?

Yes, we use both have got to, for obligation, and had better, for advice, a lot in speaking.

You've got to be careful with a cat
You'd better get something a bit quieter.
I'd better go - I mustn't miss the helicopter!

So they're not used in formal writing?

No. There's something very important about must and have to. The positive forms are very similar in meaning, but the negative forms are completely different.

You mustn't forget ...
(don't forget - you have no choice)
If you don't like him, you don't have to see him again.
(there is no obligation to see him again, but you have a choice)

Umm, I'm still a bit confused ...

Here's an example you can remember:
In a non-smoking area you mustn't smoke, but in a smoking area you don't have to smoke but you can if you want to.

Ah! Right, I mustn't forget that.

No, you mustn't! OK, let's look at advice, telling people what you think is a good idea. We use should for advice, or making suggestions, and must for strong advice.
You must go for a walk with the dog at least once a day.
Maybe you should go for a coffee or lunch and see how you feel?
You shouldn't leave it on the street.
I think I've heard people use should in other ways, like ' he should be here in a minute' - that's not advice, is it?

No, that's talking about what is likely or probable. We'll look at that use another day. We use modal verbs in different ways.

I see. So I should think about one use at a time.

Yes, exactly. You mustn't get confused by too many uses at once.

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Have to, must and should for obligation and advice

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Oliver and Alfie visit the local pet shop, where they are surprised to see Amy. Meanwhile, something is on Sophie's mind ...

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Wordwall - Crea mejores lecciones de forma más rápida

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Cuestionario - Una serie de preguntas de opción múltiple. Pulsa la respuesta correcta para continuar.

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Exercises Explanation Downloads

Modal verbs of deduction - must, may, might, could, can't

Exercise 1

Choose all the correct modal verbs of deduction for each gap below. In some sentences there are TWO possible correct answers.

Modal verbs of deduction - Grammar chart

May, might

We use may and might to talk about things that are possibly true, but we don't know for sure.

He says Betty is his friend, but I think she may be his girlfriend.

We use may not or might not to talk about things that are possibly not true, but we don't know for sure.

You should call her. She might not know where you are. (=perhaps she doesn't know)

can

We don't use can as a modal of deduction.

He might/may be at home now. (NOT He can be at home now)

Must, can't

We use must when we are sure, or quite sure, that something is true.

But we use can't (NOT mustn't) when we are sure, or quite sure, that something is not true.

We've been walking for hours. It can't be far from here. (NOT It mustn't be far)

+ be -ing

After may, might, must or can't we can use be + -ing, when we are talking about actions in progress.

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Modal verbs of deduction - must, may, might, could, can't - Test-English

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Modal verbs of deduction - must, might, could, can't. Modal verbs to talk about deduction and probability. English intermediate grammar exercises.

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