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'can' and 'could'

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.

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can and could: other uses 2

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Comments

Hi Orionne,

It's not an entirely correct sentence, I'm afraid.  I think a better version, trying to keep as close to your version as possible, would be something like:

I can imagine how various kinds of sarcasm WOULD be preponderant on existing social networking sites in place of vulgar remarks and nasty comments.

Best wishes,

 

Peter

The LearnEnglish Team

Hello, I have a question about impossibility in the past:
We use the negative can’t or cannot to show that something is impossible.We use couldn’t/could not to talk about the past:
He was obviously joking. He could not be serious.
What if I say: "He was obviously joking. He couldn't have been serious". What is the difference between "he couldn't be serious" and "he couldn't have been serious" if we are talking about impossibility in the past? To me "couldn't be" in the sentence "He was obviously joking. He could not be serious" seems like he tried, but failed to be serious (ability during a specific event) or something like "he couldn't be serious when he was a little baby" (impossibility in general).

Hello Coldfan,

In this context I would say that the two forms are intechangeable:

He was obviously joking. He couldn't be serious.

He was obviously joking. He couldn't have been serious.

However, there is a difference in terms of range of meaning.  We can use 'could' for both general and specific ability in the past.  For example:

He couldn't swim. [general ability - he didn't know how]

He couldn't swim because he was tired. [specific ability in a particular case]

However, we can only use 'couldn't have' for particular situations, not general ability.

I hope that helps to clarify it for you.

Best wishes,

 

Peter

The LearnEnglish Team

Thanks for the reply, Peter.
As far as I understood, the sentence "He was obviously joking. He couldn't be serious" may mean that he didn't have the specific ability in a particular situation. For example, he was trying to be serious, but failed though. On the other hand, the lack of the general ability of being serious is also possible. For example, he was born like this.
Am I right?
 

Hello Coldfan,

Yes, that's correct, though I think the idea that somebody is born without the ability to be not serious is fairly unlikely!  Therefore, though 'couldn't' can be used for general and specific ability, the context of the sentence make is clear that the speaker is almost certainly talking about a specific situation rather than a general inability.

Best wishes,

 

Peter

The LearnEnglish Team

Sir,
I have come to know that we use can and could also for the possibility (mostly we use may or might).
Ex:- Interviewer to Candidate: You could go now.
My question is that while we are talking about possibility, when do we need to use may, might, can, could, would?
please explain with some examples......
Thanks and Regards
KRISHNA

Hello Krishna,

That's quite a detailed question, with five different modals in one go!  Fortunately, we've got just the page you need, which looks at how we use modal verbs to talk about things that are certain, probably and possible, gives examples and explanations, and also provides an exercise to practise them.  You can find the page here.

Best wishes,

 

Peter

The LearnEnglish Team

Hi peter,
Thanks a lot for the help.

Hi Team,
Is it possible to use "did i anything wrong" because usually we use it as "did i do anything wrong".
Could you please help me on this.
Thank you 

Hello sanover,

To form the question it is necessary to use the auxiliary verb, so 'Did I do...?' is the correct form.

Best wishes,

 

Peter

The LearnEnglish Team

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