Level: beginner

Possibility

We use may, might and could to say that something is possible, but not certain:

They may come by car. (= Maybe they will come by car.)
They might be at home. (= Maybe they are at home.)
If we don't hurry, we could be late. (= Maybe we will be late.)

We use can to make general statements about what is possible:

It can be very cold here in winter. (= It is sometimes very cold here in winter.)
You can easily get lost in this town. (= People often get lost in this town.)

Be careful!

We do not use can to talk about specific events:

A: Where's John?
B: I'm not sure. He may/might/could be 
(NOT can) in his office.

Notice the difference in meaning between can and may/might/could:

That dog can be dangerous.
(= Sometimes that dog is dangerous. I know.)

That dog may/might/could be dangerous.
(= Perhaps that dog is dangerous. I don't know.)

can and may/might/could

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Level: intermediate

We use may have, might have or could have to make guesses about the past:

I haven't received your letter. It may have got lost in the post.
It's ten o'clock. They might have arrived by now.
Where are they? They could have got lost.

We use could to make general statements about the past:

It could be very cold there in winter. (= It was sometimes very cold there in winter.)
You could easily get lost in that town. (= People often got lost in that town.)

could and could have

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Impossibility

Level: beginner

We use can't or cannot to say that something is impossible:

That can't be true.
You cannot be serious.

Level: intermediate

We use can't have or couldn't have to say that a past event was impossible:

They know the way here. They can't have got lost!
If Jones was at work until six, he couldn't have done the murder.

Certainty

Level: beginner

We use must to show we are sure something is true and we have reasons for our belief:

It's getting dark. It must be quite late.
You haven’t eaten all day. You must be hungry.

We use should to suggest something is true and we have reasons for our suggestion:

Ask Miranda. She should know.
It's nearly six o'clock. They should arrive soon.

Level: intermediate

We use must have and should have for the past:

They hadn't eaten all day. They must have been hungry.
You look happy. You must have heard the good news.
It's nearly eleven o'clock. They should have arrived by now.

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Comments

Hello abdullah,

You can find an explanation of 'can' and 'could' on our can, could and could have page, and be sure to look at the other pages in the Modal verbs section as well.

Best regards,
Kirk
The LearnEnglish Team

Good morning!

Reading this article, I find myself a little surprised as I always assumed a different pattern of usage for "may", "might" and "might have" because I basically would tend to think about how a sentence sounds more than how much it adheres to grammar rules. I know it's a bad, bad habit and that's why I tend to come to the basics from time to time, although being in the upper-intermediate/advanced level. This is most likely a wrong mindset coming from (another) bad habit of trying to search a direct translation of tenses between my native language and English so a big THANK YOU for this great article.

which modal should be used in the following statement? Please give reasons as well
You_____ do your work properly.
i think so it should be 'should'

Hello meena1234,

Almost any modal verb can be used in this sentence: will, can, should, could, might, may, won't, can't, must etc. Without a context it is impossible to say which is needed.

Best wishes,

Peter

The LearnEnglish Team

Hello,
I'm a little confused about some forms of modal verbs: you say that we use the negative "couldn't" to show that something was impossible, however, some books say that for impossibility in the past we should use "can't have" or "couldn't have" (for ex. Sarah hasn't contacted me. She can't have got my message.) So, where's the truth? Thank you a lot

Hello elena24,

You are confusing two different uses here. The first is the modal 'couldn't', which can describe past impossibility:

He couldn't swim when he was six.

I couldn't stop playing the game.

We cannot substitute 'can't have' here.

The second form is the perfect modal 'can't have', which is used when we are deducing somether from evidence. It shows our belief or inference about a situation, not a fact about the past. For example, in your sentence above the speaking is deducing that Sarah can't have got the message because she has not been in touch - a logical deduction from evidence. In this use both 'couldn't have' and 'can't have' are possible.

Best wishes,

Peter

The LearnEnglish Team

 

Here is two confusing sentence...
1. If we don’t hurry we could be late.
>>>Here could be is used to shows something is possible in future.
but,
2. It could be very cold in winter.
>>>Here could be is used to describe past. how??

Hello praixx,

In 2, 'could be' seems to be expressing a possible future, just as it did in 1.

Best wishes,
Kirk
The LearnEnglish Team

Hello teachers. I have a question for you if you don't mind. Please can someone say why in this sentence the 'would' instead of 'would have'?
I thought back to our childhood, when she and I would find ourselves walking ...

Hello rewand,

This is 'would' used to describe past habits. For example, I could say 'When I was a child, I would go for long walks every morning'.

'Would have' here would describe an unreal past - something the speaker would have done (if it had been possible).

Best wishes,

Peter

The LearnEnglish Team

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