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

Thank you for such a brilliant question.Stephen Jones' reply to your query has cleared my confusion as regards the better choice between the two sentences.

where we use 'could be'

Thanks a lot for the very useful lectures.my question is how design a grammar course integrating a cultural approach.thank you.

Thanks for this course!

Thank You British Council, 

I joined today. I Have a question. How much time would I spend to learn from British Council and How much time it will take for fluent my English Writing Skills. do you have any suggestion?

What is the difference of "can" and "be able to"?

Hi sarah,
'Can' and 'be able to' are both modal verbs which are used to talk about ability. They can be used interchangeably in many situations. 'Be able to' is often used with 'will' to talk about future ability as you can't use 'can' after 'will'. 
For example, I go running a lot now and in a month I hope I will be able to run 15 kilometers without stopping.
I hope this helps.
-Erik
The LearnEnglish Team

Thanks to British Council,i am improving on my spoken English.

i think the session is very much useful  every leaners,especialy for them whose want accuracy in spoken english.very thanks to bbc.

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