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 Muhammad,
'Could not' in these sentences refers to something being not possible.  Therefore we can rephrase each sentence as follows:
 
We knew it could not be true.
We knew that it was definitely not true.
 
He was obviously joking. He could not be serious.
It was not possible that he was being serious.
 
I hope that clarifies it for you.
 
Best wishes,
 
Peter
The LearnEnglish Team

Sir Peter,
whether the meaning of "could" and "Perhaps/Maybe" is same, according to this ? If we don’t hurry we could be late. (= Perhaps/Maybe we will be late).

Hello Baloch Faisal,

Yes, that is a reasonable approximation of the meaning.

Best wishes,

 

Peter

The LearnEnglish Team

Hi Baloch Faisal,

I wouldn't say that could and perhaps or maybe have the same meaning, as they are used differently in a sentence (could is a modal verb), but it's true that all of them are used to indicate possibility. In this way, yes, "we could be late" and "perhaps we will be late" essentially mean the same thing. As Peter and I point out in response to your other question, many speakers would say "we might be late" in this situation.

Best wishes,

Kirk
The LearnEnglish Team

Can I use couldn't for possibility in the second conditional sentence? For example, If I lived in a big city, I couldn't breathe fresh air every morning.

Hello cabronasoon,

Yes, it's perfectly fine to use 'could' in this way.

Best wishes,

 

Peter

The LearnEnglish Team

HI
Can i write like this?

IF i lived in a big city, i wouldn't breathe fresh air every morning...

Thank you

Hello bharathviki,

Yes, that sentence is grammatically correct.

Best wishes,
Kirk
The LearnEnglish Team

if i am not certain about a past event
1.Then which one should i use could have+pp or might have+pp or may have+pp
2.or all mean same and can be used interchangeably.
please explain in the light of the above examples that you have used above

It’s ten o’clock. They might have arrived now.

They could have arrived hours ago.

they may have arrived by now

Hi imran_000,

In the specific context you mention, all three modal verbs structures you list mean the same thing - they can be used interchangeably.

Best wishes,
Kirk
The LearnEnglish Team

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