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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 teacher,

Thank you for your reply. As you said, and also according to a number of grammar books I have read, to say something is impossible in the present, we use 'can't'. But I sometimes see sentences using 'could (in a negative context)/could not' that seem to suggest impossibility in the present. Could you explain why 'could' is used in the following sentence?

"Nobody could take serious issue with his endorsement of the principle that police officers must be deterred from breaking the very laws they are empowered and entrusted to uphold"

Hi brian1010,

I can't be absolutely sure without knowing the full context of the sentence. But I think it's fine to use Nobody could ... to mean impossibility in the present because it uses could – not couldn't. If we make a version of the sentence using couldn't:

  • People couldn't take serious issue with ...

It isn't equivalent in meaning. It would be understood as referring to a past ability, not a present possibility. As you've seen above, could and couldn't have nuances in their meanings and aren't exact opposites, so could with a negative subject (e.g. Nobody could) isn't the same meaning as couldn't (for the meaning of possibility, at least).
 

For other meanings, could and couldn't may be more direct opposites. The sentences below about ability do have the same meaning.

  • Yesterday, nobody could answer the question.
  • Yesterday, people couldn't answer the question.

Does that make sense?

Best wishes,

Jonathan
The LearnEnglish Team

Hi teacher,

Are you referring to this section above: "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.)"

Does that mean we cannot use "could not" to convey a negative meaning in the sentences above because only "could", not "could not", can indicate a present possibility? Should we use "may not" instead if we want to convey a negative meaning?

Thank you.

Hi brian1010,

Yes, that's right. To say that those things are impossible for them to do, we can use can't or cannot, but not could not.

  • They can't come by car.
  • They can't be at home.

Using may not is possible, but the meaning is a bit different. May often indicates permission, so if we say They may not come by car, it means 'they cannot come by car because they don't have permission'. This usage of may not is also quite formal and emphatic.

Also, it might be confused with the 'not sure' meaning of may. They may not come by car (if there's no other context) would probably be understood as meaning 'I'm not sure whether they'll come by car or not', which is different from They can't come by car. So, I wouldn't recommend using may not for this meaning.

See this page for more explanation and examples about mayhttps://learnenglish.britishcouncil.org/english-grammar-reference/may-and-might

Best wishes,

Jonathan

The LearnEnglish Team

Thank you for your detailed reply. I would like to ask one more question as I just came across a sentence, which reads: "No one can say what might not happen if there were another earthquake."

I notice that the writer uses "can" rather than "could". Does that mean in the sentence I referred to earlier ("Nobody could take serious issue with his endorsement of the principle..."), "can" is also an acceptable alternative?

Hello brian1010,

Yes, you could use can in your sentence. Could has a distancing effect, making the sentence more hypothetical; can makes the sentence more immediate, as if describing a real situation. The difference is really only one of nuance, however.

I think your sentence about the earthquake may have an error. The phrase '...what might not happen...' seems odd; '...what might happen...' is more likely, I think.

 

Peter

The LearnEnglish Team

Hi teacher,

In the following two sentences, could I use "could" instead of "would"? Would there be any difference in meaning?

1. Would you lend me the car tomorrow night, Dad?
2. Would you fill in this form, please, sir?

Thanks.

Hi teachers,

Not sure if you have missed my question. I would appreciate it if you could give me a reply.

Thanks!

Hello brian1010,

Yes, somehow we missed your question -- sorry about that!

In both sentences, some might argue that 'would' speaks more of willingness and 'could' speaks more of ability, but in most cases, both forms would be correct and mean the same thing.

All the best

Kirk

The LearnEnglish Team

Could you please tell me the difference between : I might be able to come today vs I might come today

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