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

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

Hello maxmamun,

Could you please ask a more specific question? Explain to us what you understand and don't understand so we can better help you.

All the best,
Kirk
The LearnEnglish Team

Hi

While I understand the use of "could have", would there be such a thing as "can have"?

Regards,
Tim

Hello Tim,

We do not use 'can have' as a perfect modal. We do use 'can't have', however, in order to express a logical deduction of a past event:

He can't have left the building because we would have seen him.

 

Best wishes,

Peter

The LearnEnglish Team

Hello ,Sir. I hope you are doing well. My question is that you have used the usage of can (Requests or Refuse Requests) , moreover the sentence that has made me strained is , "Can I go home now?" I want to know whether this sentence shows "request" or "permission". Thanks a million in advance.

Hello nadarali1996,

Giving or asking for permission is a specific kind of request -- you are requesting someone for permission. Does that make sense?

All the best,
Kirk
The LearnEnglish Team

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