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 Team
Can we use "May" for polite offer? in the same manner we user could. such as saying : "May I help you?"
Thankyou

Sir, I have a question in mind that has been making me confused ;therefore, I want to know the difference between these two sentences. Ex. (i) I was able to ask him a question but I didn't do that. (ii) I could have asked him a question but I didn't do that.

Hello nadarali1996,

When talking about past ability we can refer to general ability or ability at a specific moment. Both 'able to' and 'could' can refer to general ability. However, it is unusual to use 'could' to refer to ability at a specific moment (a single occasion). Thus we could say:

I was able to ask a question, but I didn't.

However, we would not say:

I could ask him a questions, but I didn't.

 

In your example you have 'could have', which is slightly different. The difference in meaning in this instance is that 'was able to' does not tell us whether or not the question was asked. It may have been or it may not - the second half of the sentence is needed to tell us that it was not. On the other hand 'could have' immediately tells us that the question was not asked. 'Could have' tells us of a past possibility which was not followed. Thus we can say:

I was able to ask him a question and I did.

and

I was able to ask him a question but I did not.

or

I could have asked him a question but I did not.

but not

I could have asked him a question and I did.

 

Best wishes,

Peter

The LearnEnglish Team

can we use could and would in present/future form. Please give us examples for this.

Hello jackon1992,

It is possible to use these words with future meaning. For example:

I could visit tomorrow, I suppose.

I would go there next week if she asked.

 

Best wishes,

Peter

The LearnEnglish Team

Could you, please, explain this: They can't have left us without saying goodbye. Is this correct to use such a structure 'can't have left'?

Hello vvst78,

Yes, 'can't' + verb is commonly used to say we believe that something is not possible (e.g. 'You can't be tired - you've just woken up!') and 'can't have' + past participle is used in the same way to talk about something in the past. Take a look at the 'must as deduction' section of this BBC page for more on this.

All the best,
Kirk
The LearnEnglish Team

 

Thank you very much!

My question is , regarding the usage of could in showing past ability of something :
A) Telegrams "could be" sent through post offices in 1879.
B) Telegrams "could have been " sent through post offices in 1879.
Which one is correct ? What is the difference ?
Thank you

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