Level: beginner

Possibility and impossibility

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

They could come by car. (= Maybe they will come by car.)
They could be at home. (= Maybe they are at home.)

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.)

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 could have to make guesses about the past:

It's ten o'clock. They could 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.)

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.

Ability

Level: beginner

We use can and can't 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 and can't to talk about the ability to do something at a specific time in the present or future:

I can see you.
Help! I can't breathe.

We use could and couldn't to talk about the past:

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

Level: intermediate

We use could have to say that someone had the ability or opportunity to do something, but did not do it:

She could have learned Swahili, but she didn't want to.
I could have danced all night. [but I didn't]

Permission

Level: beginner

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.
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 for free.

We use can't to refuse permission or say that someone does not have permission:

You can't go home yet.
Students can't travel for free.

Requests

We use could you … 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

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 could give you a lift to the station.

Suggestions

We use could to make suggestions:

We could meet at the weekend.
You could eat out tonight.

Questions and negatives

We make questions by putting the subject after can/could:

Can I ...?
Could I ...?
etc.
Can you ...?
Could you ...?

 

The negative form is can't in spoken English and cannot in written English.

We sometimes say cannot, but it is very emphatic.

The negative form of could is couldn't in spoken English and could not in written English.

can and could: possibility 1

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can and could: possibility 2

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can and could: other uses 1

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can and could: other uses 2

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Comments

Hello, do these statements sound natural?
1. Yesterday I could have taken some rest, but I continued to work hard instead.
2. Though I was able to have some rest yesterday, I couldn't feel well enough to continue my work.
3. I could stop my work last year, but my parents talked me into proceeding it.
4. I could stay with you yesterday, but I had to go.
All the best,
Oleg

Hello Oleg,

The first sentence is fine.

The second sentence should say 'didn't feel' rather than 'couldn't feel'. This is because feeling well is not something we can do as we do not control it directly. We would use 'could' for something we are able to do.

The third and fourth sentences need to use 'could have' rather than 'could' as they describe unrealised past possibility.

 

Peter

The LearnEnglish Team

Thank you, Peter. So, 'I could go for a walk yesterday' is wrong cause it is ambiguous whether it was possible for me to go or I was able/managed to go. And if there was a possibility to go, I should say I could have gone, And is this the reason not to use could for possibility in the past?
All the best, Oleg

Hello Oleg,

'I could have gone for a walk yesterday' tells us that it was possible, but that you did not go.

'I could go for a walk yesterday' tells that you were able to go for a walk then, with the implication that now you cannot. You might say this if you have a broken leg and are complaining about how your life has changed, for example.

 

Peter

The LearnEnglish Team

And what if I said I could go for a walk yesterday, but today I can't cause I have got a lot of work to do. Having got a lot of work isn't skill, power or means, is it? If it is not correct how should I express this idea properly?
Oleg

Thank you Peter, for your excellent explanation. Though I think that the latter is not very typical use of could. Sometimes it's not easy to define what could means. For example: 'Sure, he got finished in the third round, but what made this match up different was the fact that Khabib actually looked like he could be in trouble.'
Do you think was able to get into trouble could be used!
Oleg

Hello Oleg,

We only use 'be able to' to describe ability. Since getting into trouble is not something Khabib wants to do or is trying to do, but rather something that happens to him in spite of his best efforts, it is not appropriate in that sentence.

 

Peter

The LearnEnglish Team

Thank you, Peter.
Oleg

Hello,

Hello. Could you please view upon this. Would 'the burglar could get in through the window' followed by 'but later he changed his qualification for pickpocket' be correct? ) I mean he had a general ability to get through the windows, not the one-moment action.
All the best,
Oleg

Thanks for your kind answer, Peter.)
Please, I am just wondering if the below statements sound English and express well different ideas:
1. I said to the detective that the burglar could get in through the window.
2. I am sure that the burglar could easily get in through this window.
3. I have/had no doubt that the burglar succeeded in getting/managed to get into the house through this window.

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