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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.

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

Hallo, your lesson is very helpful, but there is still one unclear situation for me:)
Could you explain to me what the difference is between "could" and "could have" when we want to express possibility in the past?
for example:
I don't know that could have been John. 
I don't know that could be John. 
In both cases i imply, that I'm not sure if it was John.
So which one is right and why?:)
Any help would be much appreciated, thanks in advance:)

Hello monnzz,
The sentences need a little addition:
(1) 'I don't know if that could have been John.'
and
(2) 'I don't know if that could be John.'
(1) is about the past: we are considering (doubting) if it was possible for what we saw to be John, perhaps because it did not look like him, perhaps because John is on holiday etc.
(2) is actually about the present: we would usually use this when we can still see the evidence, such as a photo from the night before, and are discussing it.
An alternative would be to use a past simple form, and I think this is probably the most natural way to say it: I don't know if it was John.
I hope that clarifies it for you.
Best wishes,
 
Peter
The LearnEnglish Team

First of all I'd like to thank you for your answer.
Unfortunately, not exactly:(
One more example would be much appreciated:)
1) It could have rained. 
2) it could rain.
I'd like to underline we are still talking about the past possibility.
So 1) means,we don't know if it rained or not, but from what we can see now (things are wet, for example) it is one possibility.' (I stole your words here from topic below, sorry, but this example is great:))
But what about the second one? What does it mean?
Sorry for bothering you:(
P.S.  In your answer i noticed an interesting usage of 'would'. (we would usually use this when we can still see the evidence). But couldn't  you just say 'we usually use...'. 
So far I know that we can use 'would' for the second conditional, somebody's opinion, presumption, polite requests, uncertainty or past form of will.
But i wouldn't say that even one of my row suits your usage.
Or is it the second conditional, and you imply that if we used 'could' we would use it for 'here goes some explanation'. But at that moment when it was present time we didn't use 'could' so it was impossible?
Thanks in advance, Mike

Hello Mike,
As far as the use of 'would' goes, it's a use of the modal to show typical behaviour, with a similar meaning to 'we (would/will) usually choose to'.
Your example of 'It could rain' here does not appear to me to be about past possibility.  As I said in my previous answer, it appears to be (speculation) about the present.  Perhaps you could provide the context for this sentence, because at the moment it does not appear to be about the past at all.
Best wishes,
 
Peter
The LearnEnglish Team

Could you explain what's  different between "may have" and "might have", "can have" and "could have"? All of them are used in the past

Hi there,
I am not quite sure the meaning of "could" in the following sentence taken from the words of a song.
You have built your wall so high that no one could climb it but I am gonna try. 
Does the word "could" mean "would be able to" or "could" in the past?
Thanks for your help in advance.
Regards,
Chris

Hi chrisf,
The tenses in this sentence are a little inconsistent.  In standard English, we'd be likely to say either 'You built [past simple]... could climb...' or 'You have built [present perfect]... can climb...'  However, songs are a form of poetry and poetry breaks all kinds of rules, so it's not really a good idea to analyse the grammar too much.  Just enjoy the song!
Best wishes,
 
Peter
The LearnEnglish Team

hi,
 
till now i cant understand what is the difference in the meaning between the following
in the past, you could lose your way in the dark
in the past, you could have lost your way in the dark
 
is the following explanation true:
for the fist one, it means there was a pliability to lose your way in the past
for the second one, it means there was a possibility to lose your way in the past but you didn't lose it
 
thanks in advance

Hi mohanad0002220,
A good question - thank you!  Modal verbs have many different meanings, and could have is no exception: it can be used for ability, possibility and for logical deduction, for example.
When we talk about what possibility, we usually use 'could have' to talk about things that were possible but didn't happen, and 'could' to talk about things that were possible (and may or may not have happened), so what you say is correct.  Well done!
However, the sentence could also mean something different if you are talking about logical deduction, for example:
'You could have lost your way in the dark' = 'We don't know what happened, this is one possible explanation'.
This means that a simple sentence can have several possible meanings.  For example:
'It could have rained this morning' could mean...
'It looked like rain, but in the end it didn't rain.'
...or...
'We don't know if it rained or not, but from what we can see now (things are wet, for example) it is one possibility.'
As you can see, it's very much dependent on the context and the speaker's intention.  That's what makes modals so tricky, and so wonderfully useful!
Best wishes,

Peter
The LearnEnglish Team

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