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

Are there any other way to use could in present other than could be and could have?

For using could in present could be is used.for using could in past it can be used as past ability as well as could have in conditionals.for using could in future could is used for possibilty not for ability am I right

hello sir,i have gone through the explanation this section provides but i still have some doubts.1)If i say these things could kill you,does it mean that there is a possibility of these things killing me or certainty of getting killed?what would happen if we replace could with can in this statement?
2)in a question,what difference will it make if we replace could with can or visa versa?

Hello magicspeechqwerty,

In answer to your questions: 1) both 'can' and 'could' refer to a possibility in that sentence. They essentially mean the same thing, though 'can' is sometimes used for general statements of fact and 'could' for a more specific situation. 2) As the explanation indicates, 'could' is considered more polite than 'can' in requests.

Best wishes,
Kirk
The LearnEnglish Team

Hello, some time ago i met sentence: (I didn't know who I was, or what I should be doing.) Why here the 'should be' is using instead 'should have been' ? I think here is the situation when we transfer in the past and say from there as if we are in present. But i can be mistaken. Thanks.

Hi rewand,

By using 'should be', the writer is speaking from within the perspective of the past, thereby making it more real for us now. It's as if they want us to imagine that we are with that person in the past.

Best wishes,
Kirk
The LearnEnglish Team

Great answer:) You already like professional. I'm thankful.

Ok:)
1. 'He could stop smoking' - (here it is clear that person can stop smoking - the future or present situation).
2. 'He could stop smoking when he was learning at university' - (the past situation, he was able to stop)
3. 'He was able to stop smoking a two weeks ago' - (the past situation, but it is specific situation, and we can't use here the (could)).
4. 'He could have stoped smoking if he want to' - (the hypothetical situation. Our imagination about some imaginary situation - present). I doubt about using in the present.
5. 'He could have stoped smoking if he wanted to' - (the hypothetical situation. Our imagination about some imaginary situation - past).

Hello rewand,

Yes, you're understanding this now. Just a couple of notes: First, sentence 2 is a little bit odd (though not incorrect) because to say that he did stop, we'd use 'He was able to stop smoking ...' If you use sentence 2 as is, it's as if he was able to stop when he wanted to, but sometimes started again. 

Second, the fourth sentence is not correct. To make the sentence reflect your explanation in brackets, it should simply be 'He could stop smoking if he wanted to' – this sentence is a more complete form of sentence 1.

Best wishes,
Kirk
The LearnEnglish Team

Thanks) I got it now. I can't use (could have) in the present, because it is past-specific form. For example: We just finished running and i would say: - (We could have finished earlier) - specific past situation.
If this was not specific situation in the past i would have said: - (Weather could be better then)

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