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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

"If we offer you the post, when can you start?"

Could I say "when could you start" or "when may/might you start" instead?

Thanks teachers!

Hi MarciaBT,

Yes! When could you start? also works fine.

The versions with may or might are possible, but less common.

  • May often means permission, so When may you start? has a more specific meaning: 'When do you have permission to start?'. 
  • The version with might means there is more uncertainty about when the person can start. It invites the person to give a tentative answer (while the versions with can and could invite a more definite answer).

Does that make sense?

Best wishes,

Jonathan

The LearnEnglish Team

Thanks for your reply Sir.

Would "could" convey an extra sense of politeness and be better than "can" here?

Hi MarciaBT,

No problem. Yes, could shows more politeness. Since a job interview is often the first time that the people have met, that's definitely the option I would choose.

Best wishes,

Jonathan

The LearnEnglish Team

Hi teacher. Hope you are doing well! This is the first time that I have posted comments here. I find this page really useful. May I ask why "can" rather than "could" or "may" is used in the sentences below? I have a feeling that since the sentences refer to some future action, "could" or "may" would be a better choice. Please correct me if I am wrong. Thanks!

1. A compromise has to be reached between all the powerful vested interests before any restoration work in the city can take place.
2. I liaise with end-users, resellers and partners to see how we can improve our portfolio of products and services.

Hi AsahiYo20,

Can works fine in each sentence. However, the use is different in each example. The first sentence uses can to express possibility. The second uses can to express ability. This affects whether or not could can be used.

 

Even though if is not used, the first sentence is a form of conditional, expressing a condition and a dependent result. Could would make the sentences hypothetical and would require changes in the first clause:

A compromise would have to be reached between all the powerful vested interests before any restoration work in the city could take place.

[the speaker thinks the situation is unlikely]

 

In the second sentence, could would have a past meaning (past ability), and so the verb in the first clause would also need to be past:

I liaised with end-users, resellers and partners to see how we could improve our portfolio of products and services.

 

Peter

The LearnEnglish Team

Thanks for your detailed reply!

But am I correct to say that those two sentences refer to express future possibility and future ability respectively? If that is the case, I am confused about why "can" is used because I have the belief that "can" should not be used to talk about future possibility (expressed by could, may, might) or future ability (expressed by will be able to).

Hello again AsahiYo20,

The first sentence does not express future possibility but rather describes the present situation. It could be an answer to a question such as 'Why isn't the restoration work being done?', for example.

 

The second sentence is also not about the future. It describes a person's job in general terms, not a particular future event. It would be an answer to a question such as 'What does your job entail?' and not 'What are you doing next week?'

 

Peter

The LearnEnglish Team

Hi teachers,

1. Her colleagues could not fault her dedication to the job. --> Assuming that this sentence is not talking about the past, could I change "could not fault" to "cannot fault"?

2. The doctor examined her carefully but could find nothing wrong. --> could I delete "could" in this sentence? (i.e. ...but found nothing wrong.)

Thank you.

Hi patph0510,

1. 'Could' is referring to the past in this sentence. To say something is impossible in the present, we use 'can't' – but not 'couldn't' (see the first section above for some examples). So, your sentence with 'cannot fault' is the correct one for the present meaning.

2. Yes, you could. The meaning would be very similar, but with one difference: the version with 'could' includes the meaning of the doctor attempting or making effort to find the problem. The version without 'could' doesn't include that, and is focused on the end result (finding nothing wrong).

Best wishes,

Jonathan
The LearnEnglish Team

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