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'can' and 'could'

Undefined

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 Sir,
1) kill him before he can kill you.
2) kill him before he kills you.
Is there any difference between these two sentences. If yes then what meaning two sentences convey.
Thank you.

Hello Abhinav,

In most contexts, the sentences can be used interchangeably.

Can carries the meaning of 'has the ability to', so you could imagine a situation in which you want to stop a person getting the ability to kill. For example, you might want to act so that someone is prevented from getting hold of a weapon which would make it possible for him to kill you.

 

Peter

The LearnEnglish Team

I can't find the function related to these uses of CAN:

You can leave your hat on that chair
He won't love you as much as I can
I think he's angry with you but you can call him to check it out.

Hello Dayan

In the first sentence, 'can' seems to be used to give permission to someone. The second sentence seems a little odd to me (I think I'd say 'do' instead of 'can'), but I suppose it's describing an ability. In the third sentence, I'd say it's speaking about possibility.

Hope that helps!

All the best

Kirk

The LearnEnglish Team

Dear Sir,
I have just registered at LearnEnglish. I am so happy that I can get my doubts clarified by the full-fledged teachers who are so responsive to whoever is in need of clear and precise answer to their questions concerned with grammar. You in fact make boring and confusing grammar interesting and easy to learn.
My question is,
You have said that 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.

In the first sentence above, it's understandable that "can't have" is used as the verb in its preceding sentence is in the present. So is in the case of the second sentence.
What if we used "can't have" when we talk about a past action. For instance,
I heard someone knock on the door.It can't have been our neighbour.
Thank you.

Hello GIRIKUMAR,

 

While could can function as the past form of can when describing ability (I can swim vs I could swim), here you are using them for (im)possibility. In this use, they are interchangeable in many contexts. However, can't have is generally used for more recent contexts and couldn't have for ones further back in time.

 

You can read more about this in the discussion on this page:

https://english.stackexchange.com/questions/141589/cant-have-been-vs-couldnt-have-been

 

Peter

The LearnEnglish Team

I don't think I will be wanting for any better explanation than that of yours. Thank u so much, sir.

Thank you, Kirk! Your comment seems to deliver invaluable info about the use of could in these examples!)
And not only for me. Would you think so?
Oleg

Hi again. Do you think these statements sound pretty good:
- I say that I can swim, and I could help you to cross the river.
- I said that I could swim, and I could/would help him to cross the river.

All the best,
Oleg

Hello Oleg

Yes, those are both grammatically correct. I'd suggest repeating 'that' in the second one: 'I said that I could swim and that I could help him to cross the river'.

All the best

Kirk

The LearnEnglish Team

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