can, could and could have


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.




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,
The LearnEnglish Team

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

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,
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)

Hello Mr. Kirk. Greetings.
Surely i understand that (should/would/must) - don't have the past forms. But as you have said they
can be used as in the past as and in other tenses. I mean the verbs: (could/should/would/must) don't have some specific time. We use them whether in the past or in the present or future, to express FACT, ASSERTION.
If we add to them the perfect-aspect (could/should/would/must have) - they mean the "imposible condition". It's used to describe a situation that didn't happen, and to imagine the result of this situation. Whether in the past or present or future to express SUPPOSITION, HYPOTHETICAL.

Hello rewand,

I'm afraid you're still trying to oversimplify the use here and the rules you are trying to impose on the language do not fit the way it is used.

'He could stop smoking' - a future possibility

'He could have stopped smoking' - a possibility in the past; without further information we do not know whether or not he stopped; we are simply stating that it is not impossible

Modality is very much context-dependent and describes how the speaker sees a particular action or state.

Best wishes,


The LearnEnglish Team

Can the (should/would/must) - use in the past? Or i should use in the past only (should/would/must have)? Thanks)

Hello rewand,

I'm afraid I can't answer this question completely, as it's quite complex and would take too long to explain. These modals definitely don't have past forms, but can be used to express past meanings (e.g. in reported speech, 'He told me I should study more'), among other uses. I'd suggest studying and practising each modal verb in some depth – in that way you'll come across some of these uses.

Best wishes,
The LearnEnglish Team