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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

Hi sir can you help me with this sentence? You couldn't possibly love me as much i do

Hi Monty Stevens,
I'll be happy to help you, but I'm not sure what the problem is!  The sentence is correct, but has a rather odd meaning - it sounds like the speaker is saying that they love themselves more than the other person could, which is rather egotistical to say the least!
Best wishes,
 
Peter
The LearnEnglish Team

Hallo, Peter, thanks for having patience with me and answering my questions:)
what about this example:
here is some background: "Me and some friends went camping and the next morning we discovered that one of our friends missing, so
Our friend could lose his way in the dark.
Our friend could have lost his way in the dark.
 
Honestly, Im not sure this example is what we need in this particular topic, because i believe i need to use "could have lost" here, because of that is one of explanations why our friend lost his way. (perhaps, he didn't like camping and went home not notifying us)
So is it possible to remake this situation somehow so we could use "could lose" instead? 
 
P.S. So we can use "would" to show typical behaviour such as, for example:
I would usually go for a walk in the evening.
(But could i just say: I usually go for a walk in the evening?) Does it mean the same? 
 

Hello monnzz,
In your first example I think 'could have' is necessary.  If we weren't talking about a specific situation but rather indulging in general hypothetical speculation about the possibility of getting lost then 'could' would be possible.
In the example you give 'would' describes a past habit, similar to 'used to', rather than a present habit.  The use of 'would' in my sentence is quite a tricky one, and it usually appears when we contrast characteristic behaviour or choices of different people.  For example:
People in my country like to go for a walk in the morning.
Oh, really?  In my country people would usually go for walks in the evening.
As I said, this use usually carries a meaning of 'would choose/like/prefer/opt for'.
Best wishes,
 
Peter
The LearnEnglish Team

Hello Peter, now I've caught the idea:) Thank you very much for such a good explanation:) 

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

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