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

Hello B H A R T I,

'be' is a bare infinitive, which is the form used after 'can' and other modal verbs. 

All the best,
Kirk
The LearnEnglish Team

Hello sir, will you tell me the difference between these two sentences
1 I will there
2 I will be there
and when we used 'be'.

Hello Bharti,

Sentence 1 is not correct. 'will' is a modal verb (like 'can' and 'could') and so in complete sentences it must be followed by a verb in the bare infinitive form. 'be' is one such bare infinitive; other bare infinitives (e.g. 'go', 'visit', etc.) are also possible there.

Best wishes,
Kirk
The LearnEnglish Team

Hello sir, could you send me some links on bare infintive 'be' uses ?.and also I want to know how can I improve my english,I can not speak english well.If there is any grammatical mistake then please let me know.

Hello B H A R T I,

For tips on how to improve your English, try our Help page.

For guidance on different aspects of the verb system in English you could look at our Verbs grammar page. However, we don't have pages specifically on the bare infinitive 'be'. The bare infinitive is used in a huge number of forms and it would pointless to list them all - there are too many. It is not an organising principle of grammar. You can find links on the page mentioned above to many different forms which are followed by the bare infinitive, such as modal verbs, for example, or forms which may include 'be', such as some passive forms.

I hope that is helpful to you.

Best wishes,

Peter

The LearnEnglish Team

Hello sir,
What is the difference between
1.simple past and
2.past perfect tense
Please clear me where to use past perfect ?
Tank you

Hello Ajaz ajju,

We have several pages on this topic.

For information on the past simple - here.

For information on the past perfect - here and here.

Best wishes,

Peter

The LearnEnglish Team

Hello I write this in order for you to help me clarify this. what is the difference between these following sentences: Let the whole wold should be singing and Let whole world should sing.

Hello Lamastry,

I'm afraid that neither of these clauses are grammatical in standard English. Perhaps 'Let the whole word sing'?

Best wishes,
Kirk
The LearnEnglish Team

Thank you

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