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

Thank you so much!

Thanks to all teachers of this site. Not long time ago i asked about difference between (could have) and (could) the both these are in the past. And i obtained the answer that the ('could have' is used to speak about a specific instance of being able to do something, but not doing it.). And ('could' refers to a very general past ability, not to a specific situation.) May i ask you to give me the example of the (general past ability) and (specific past ability) with using of COULD HAVE and COULD as in the past. Thanks beforehand)

Hello rewand,

There are a couple of examples above of 'could' used to speak about a past ability, e.g. 'I could run very fast when I was younger'. 'could have' can be used in many ways, but what I was referring to based on your question is how it can be used to talk about something unrealised, i.e. something that did not occur despite being possible, e.g. 'She could have been a great runner if she had wanted to'.

If you want to practice or understand this topic more, I'm sure you can find plenty of useful resources by doing an internet search for something like 'could and could have'.

Best wishes,
Kirk
The LearnEnglish Team

Thanks to you Mr. Kirk for your help. I will try your advices.

Hello and my greetings to all teachers of this site. You are doing a great job. I have read about the using of (should/could have) but i don't get the difference between (could help and could have helped). I mean i know that the first is in the present or the future. But sometimes i see the (could) in the past too. I have a question. What is the difference between the two sentences below:
Yesterday she had a trouble and i could have helped her (but i didn't)
Yesterday she had a trouble and i could help her (but i didn't)
Do these two sentences have the same meaning or there is some difference?

Hello rewand,

Thanks for your feedback! It's always nice to know that people appreciate our work. I think our ability, permission, requests and advice page – in particular, the Ability section – might help you understand this.

Your first sentence is correct, but the second is not because 'could have' is used to speak about a specific instance of being able to do something, but not doing it. 'could' refers to a very general past ability, not to a specific situation. Since your sentences refer to a specific situation, 'could' is not appropriate there.

I hope this helps you.

Best wishes,
Kirk
The LearnEnglish Team

 

Does "could" mean "can probably" here? Or maybe a hypothetical?

The comentator could have said "the game is underway".
I could have said "I will do my best".

Hello akatsuki,

These look like they refer to the past possibility to me, though it's hard to say without knowing the context. Context is key when it comes to understanding verb forms. 

Best wishes,
Kirk
The LearnEnglish Team

Hi
What does "arrived" representing in could have structure?
Also Above question
Gramatically correct?

Hi Muhammad Salik,

You can find the meanings of individual words like 'arrived' by using the Cambridge Dictionaries Online tool on the right of the page. Simply type the word into the window and click 'Look it up!' to get a definition, examples, synonyms, pronunciation and more. It's a great tool.

Your question is not grammatically correct. The best way to phrase it would be:

What does "arrived" mean in the 'could have' structure in the example above?

Have you tried our section 'Elementary Podcasts'? I think they may be very helpful for you.

Best wishes,

Peter

The LearnEnglish Team

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