The modal verbs are can, could, may, might, mustshall, should, will and would.

The modals are used to show that we believe something is certain, probable or possible:

Possibility:

We use the modals could, might and may to show that something is possible in the future, but not certain:

They might come later. (= Perhaps/Maybe they will come later.)
They may come by car. (= Perhaps/Maybe they will come by car.)
If we don’t hurry we could be late. (= Perhaps/Maybe we will be late)

We use could have, might have and may have to show that something was possible now or at some time in the past:

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

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

It can be very cold in winter. (= It is sometimes very cold in winter)
You can easily lose your way in the dark. (= People often lose their way in the dark)

We use the modal could as the past tense of can:

It could be very cold in winter. (= Sometimes it was very cold in winter.)
You could lose your way in the dark. (= People often lost their way in the dark)

Impossibility:

We use the negative can’t or cannot to show that something is impossible:

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

We use couldn’t/could not to talk about the past:

We knew it could not be true.
He was obviously joking. He could not be serious.

Probability:

We use the modal must to show we are sure something to be 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 must 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.

We use the modal should to suggest that something is true or will be true in the future, and to show you have reasons for your suggestion:

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

We use should have to talk about the past:

It's nearly eleven o'clock. They should have arrived by now.

Exercise

Section: 

Comments

Hi imran_000,

I'm afraid it's difficult to explain why this is - languages evolve over time in an organic manner, and that's just how English has developed. As far as I know, all natural languages have redundant forms.

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

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).

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

Hello Baloch Faisal,

Yes, that is a reasonable approximation of the meaning.

Best wishes,

 

Peter

The LearnEnglish Team

Hello,
I can't get meaning of following sentences-
We knew it could not be true.
He was obviously joking. He could not be serious.
Please help me overcome
Thanks a lot
Anis

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

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