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
i'm  new member please any body  could me?

Hello, and thank you for your great grammar service.
I am confused by these examples:
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)

I thought that "could" is used for conditionals. I mean that what I read above is that every winter can be very cold (not just in the past), and that I can lose my way in the dark even in the future. Am I totally wrong?
Thank you for your attention.
Regards.
Daniele Giacomini
 

Hello dg7!
 
These sentences are both past. For example, in Europe in the 17th and 18th century, there was a "mini ice age" when the weather was unusually cold.
Back then, it could be very cold in winter.
 
Likewise, for the second sentence:
In the past, there were no electric street lights. You could lose your way in the dark.

That's all - it's difficult to understand without the context of the sentences, but you can use this structure to talk about the past.
 
Regards
 
Jeremy Bee
The LearnEnglish Team
 

Hi! well my question is this, Can we use Must not instead of Can not or Could Not
for impossible actions.

Hello Farrukh!
 
Grammatically, you can, but mustn't with this meaning is uncommon these days. Generally, we use can't.

Hope that helps!
 
Regards
 
Jeremy Bee
The LearnEnglish Team

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thanks a lot  

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