Questions and negatives:

We make questions by putting the subject after can/could:

Can I …? Can you …? Could I … Could you …? and so on.

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.
We sometimes say could not.

We use can and can’t :

  • To talk about ability:

Maria can speak four languages.
I can’t swim, but my sister can.

  • To say that something is possible or impossible:

Learning English can be difficult [= Learning English is sometimes difficult.]
Children can be very naughty [= Children are sometimes very naughty.]
It’s still light. It can’t be bedtime.
 

  • For requests and refusals of requests

Can I go home now?
You can go whenever you like.
You can borrow the car today, but you can’t have it tomorrow.

  • To offer to help someone:

Can I help you?
Can I carry that bag for you?

We use could and couldn’t as the past tense of can/can’t:

  • To talk about ability:

I could run very fast when I was younger.
She couldn’t get a job anywhere.

  • To say that something was possible or impossible:

Our teacher could be very strict when we were at school. [= Some teachers were very strict.]
People could starve in those days. [= People sometimes starved.]
You couldn’t use computers in the nineteenth century.

  • To make a polite request:

Could I go now please?
Could you lend me a dictionary please?

  • To make a polite offer:

Could I give you a lift?
I could carry that for you.

We use could have:

  • to show that something is possible now or was possible at some time in the past:

It’s ten o’clock. They could have arrived now.
They could have arrived hours ago.

 

Exercise

Section: 

Comments

Hello Param,

Yes, you can say 'you could have let me know' or 'you could have told me'. They mean the same thing.

All the best,
Kirk
The LearnEnglish Team

My sister has done something terrible and i m telling my friend ''i had no idea she would be trouble'' then my friend says'' there was no way u could have seen it''

My confusion is isnt there should be '' there was no way u could see it''

Please explain which one is correct? When to use which form? Thanks Peter m

Hello maxamun,

This is essentially the same question you asked previously, and my answer is also the same. We use 'could have' to refer to actions before the event and 'could' to refer to actions before or after. In other words 'could' has a wider meaning.

 

Best wishes,

Peter

The LearnEnglish Team

Hello sir

My sister disappeared today so we have looked for her today but we couldn't find her.. now my dad just said..''there was nothing anyone could have done because her mind was set

I think it should be '' there was nothing anyone could do''

Sir please explain which form should i use and probably explain both with eample for me...thank you peter m sir in advance

Hello maxamun,

Both 'could do' and 'could have done' are possible here but 'could have done' has a more limited meaning. 'Could do' suggests a response either before or after the event: there was nothing possible to prevent it and nothing possible in response to it. 'Could have done' describes a response before the event: there was nothing possible to prevent it.

 

Best wishes,

Peter

The LearnEnglish Team

I have just said something to my friend today..and he said why couldn't you have said that yesterday?...

It should be why couldn't you say that yesterday?

Please tell me when to use which form

Hello maxamun,

In this context I don't think there is any difference in meaning. Both 'couldn't say' and 'couldn't have said' will be understood as complaints, implying that the speaker is annoyed at the person not speaking yesterday.

Generally, there is a difference. Compare:

I couldn't have said it yesterday.

and

I couldn't say it yesterday.

 

The first example is telling us that it is logically impossible for the person to have said it. It is a kind of deduction: it can't be true that I said it yesterday because I was asleep all day.

The second sentence is simply a statement of fact that something was not possible: I couldn't say it yesterday because I was not allowed by my boss.

The distinction can be quite subtle but one is about drawing a logical conclusion about the past while the other is about ability or permission in the past.

 

Best wishes,

Peter

The LearnEnglish

My friend told me to do something yesterday but i didn't...so today he is asking me '' why couldn't you have done that yesterday?

I think it should be '' why couldn't you do that yesterday ''

Please tell me when to use which form...thnank you

Hello maxmamun,

For the situation you describe, I'd say the most natural question would be 'Why didn't you do that yesterday?' If you want to use one of the two alternatives you ask about, I agree with you -- 'couldn't' is better.

All the best,
Kirk
The LearnEnglish Team

Why couldn't you have said that before/yesterday?

Why couldn't you say this before/ yesterday?

Sir please explain in very detail..every perspective

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