Possibility

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 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)

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

If we don’t hurry we could be late. (=Perhaps/Maybe we will be late)

We use could have to show that something is/was possible now or at some time in the past:

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

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.

Ability:

We use can to talk about someone’s skill or general abilities:

She can speak several languages.
He can swim like a fish.
They can’t dance very well.

We use can to talk about the ability to do something at a given time in the present or future:

You can make a lot of money if you are lucky.
Help. I can’t breathe.
They can run but they can’t hide.

We use could to talk about past time:

She could speak several languages.
They couldn’t dance very well.

 

Permission:

We use can to ask for permission to do something:

Can I ask a question, please?
Can we go home now?

could is more formal and polite than can:

Could I ask a question please?
Could we go home now?

We use can to give permission:

You can go home now if you like.
You can borrow my pen if you like.

We use can to say that someone has permission to do something:

We can go out whenever we want.
Students can travel free.

Instructions and requests:

We use could you and as a polite way of telling or asking someone to do something:

Could you take a message please?
Could I have my bill please?

can is less polite:

Can you take a message please?

Offers and invitations:

We use can I … to make offers:

Can I help you?
Can I do that for you?

We sometimes say I can ... or I could ... to make an offer:

I can do that for you if you like.
I can give you a lift to the station.

 

Exercise

Section: 

Comments

Hi,

Pardon me if my following questions seem a bit superfluous, but I would appreciate it if you could confirm my understanding on the following: You mentioned under "Possibility", that could acts as the past tense of can. You then mention under "impossibility" and "ability" that could is used to talk about the past. By this, you simply mean that could serves as the past tense of can under "impossibility" and "ability"?

Also, my second question would be that under "possibility", you mentioned the use of could have to "show that something is/was possible now or at some time in the past". Quoting your example above, I suppose I could simply add a "not", as in "could not have arrived" to mean something is/was impossible, now or at some time in the past, and hence relegate this to be under the "impossibility" section?

Thanks!

Regards,
Tim

Hello Tim,

Each modal verb can represent multiple meanings. For example, 'could' can describe ability in the past:

When I was younger I could run 10km without any trouble.

It can also describe possibility in the present or the future:

The guests could be waiting for us already.

The guests could be here before 6.00.

You need to bear in mind that one modal has several uses/meanings and not try to fit all examples into one use.

This is why the modals section on the site is organised in the way it is, with sections showing different ways to express certain notions (possibility, obligation etc) and sections showing the different uses of certain modals (can, could etc).

 

The example you give of 'couldn't have' does represent impossibility from the point of view of the speaker. It is a form of deduction: the speaker does not know for sure but is convinced that it is not possible.

 

Best wishes,

Peter

The LearnEnglish Team

Hi Peter,

Thanks for the advice. Actually, I wanted to seek your clarification on the use of "could" to talk about the past, specifically under the sections of Possibility and Impossibility. Under Possibility, I note that Could may act as the past tense of Can. Under Impossibility, you mentioned couldn't/could not is used to "talk about the past" - does this mean that couldn't/could not is acting as the past tense of cannot/can't (e.g. He was obviously joking. He Could Not be serious)?

Thanks once again!

Regards,
Tim

Hi TIm,

Yes, that is correct. We phrase it quite cautiously on the page as we do not wish to suggest one-to-one correlations in this area when concepts such as possibility, deduced probability and so on overlap a lot and have very subtle distinctions between them.

 

Best wishes,

Peter

The LearnEnglish Team

could you please tell me the difference between two sentences which are :

_ i will wait for you in the airport
_ i will be waiting for you in the airport

thank you

Hello ramzipure114,

That depends a bit on the context, but in general they mean the same thing. The difference is in how we imagine the future event. The first is a general statement, or it could also be a promise or a plan you've just decided. In the second, that time in the airport is seen as an event with some duration and you see yourself in it.

It can be difficult to see what the difference is -- there isn't any easy rule to learn. I'd recommend you pay attention for the future continuous form ('will be waiting') as you read and listen to English. As you see it more and more in context, I think you'll understand how it is used better.

All the best,
Kirk
The LearnEnglish Team

"So that I can meet you with my friends" is it right?

Hello ankitrawat18,

That is grammatically correct, though it's not a complete sentence. That doesn't matter if it's a response to someone's question, especially in speaking, but in writing it could be problematic depending on the kind of text it's used in.

All the best,
Kirk
The LearnEnglish Team

Hi !
I have to use in this sentence a modal verb "it's not possible that he heard about it on the radio. He never listen to the news.".
I wrote " he couldn't have heard about it......." But apparently it's wrong. Could someone explain me why, please?

Hi Gager,

As far as I can see 'could have' would be fine as a way of expressing that meaning. If you want to know why it was not accepted then you will need to speak to the person who made that decision.

 

Best wishes,

Peter

The LearnEnglish Team

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