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Grammar: differences between could and can

Level: Starter/beginner, Elementary, Pre-intermediate, Intermediate, Upper intermediate, Advanced Type: Reference material

An article discussing the differences between 'could' and 'can' when expressing possibility.


How can I explain the difference between 'could' and 'can' when expressing possibility? Most of the time it is clear from the context, but use of can can also express possibility (rather than ability). For example, giving advice answering the question: How can/could I improve my English? You can/could listen to the radio, watch TV and read the newspaper. Both are possible. Students want to know when to use 'could' and when to use 'can'.

Caroline Talbot

Taking your examples first, I think the short answer is that: You could listen to the radio emphasizes that this is a suggestion or piece of advice, whereas You can listen to the radio emphasizes simply that this is an option that's available. 

You could listen to the radio contains more personal involvement and subjectivity; You can listen to the radiois more strictly factual and objective. 

Similarly, How could I improve my English? is more a request for advice, whereas How can I improve my English?is more a factual question about available options. (But of course we can also answer this question by giving advice.)

Can and could, like the other modal verbs, have developed quite a range of meanings and uses. You ask how to explain the difference, and explanation can certainly help, but learning all the ins and outs of these verbs is a long process which requires plenty of experience, observation and experiment.

I think your question touches on two main issues: possibility vs. ability, and can vs. could.


1) Ability and possibility

Ability and possibility are similar ideas. If you've got the ability to do something, then it's possible for you to do it - in principle at least, although there might be something that prevents you. And, conversely, if you haven't got the ability to do something, then it isn't possible for you to do it. Both can and could (and other modals, especially may and might) are used to express various kinds of possibility, ability, permission and potential.


2) Can and Could

Could, of course, functions as the past tense of can, and like other past forms, it sometimes simply indicates past time: In those days there was no security and anybody could walk in, day or night.

But, like the past forms of other verbs, it can also indicate things like tentativeness, indirectness, deference and a wish not to impose. Compare:

I wanted to have a word with you (now)
I wondered if you needed any help (now)

with:

I want to have a word with you
I wonder if you need any help

The last two examples are more direct; the first two are more tentative, and possibly more polite – though that depends on other factors, too.

When we make suggestions, we often like to be rather tentative, so as to avoid giving the impression that we necessarily expect people to do as we say. Of course, it's possible to make much stronger suggestions – e.g. I think you should listen to the radio – but we generally prefer to adopt a more 'take it or leave it' approach, and lessen the possibility of offence on either side. So that's why You could ..... is so commonly used in suggestions (and, by the same token, Could you .....? is often used for requests). In fact, it's so common that it makes sense to learn it as one of the standard formulae for giving advice and making suggestions (and to reserve You can ....., in similar contexts, for factual statements). And, by the way, it's quite common to add always: Well, you could always listen to the radio. Of course, the well at the beginning isn't necessary, but it also contributes to the general impression of tentativeness.   

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Readers' comments (41)

  • that was really helpful discussion
    thanks
    www.blogkise.blogspot.com

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  • That's very much useful. thank you

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  • thanks . it is very effective and helpfull

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  • Hi, i have a question...
    If you are engaged with other plans, just carry on and let me know by what time you CAN or COULD make it... which one is correct can or could? Often i am getting confused with them...

    Thanks...

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  • Thank you Macmillan for providing the materials not only for myself, but in order to share knowledge with the PARENTS, my teaching staff, and of course excellent reading and review for the STUDENTS.

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  • Seems complicated when I am looking for a quick an easy to explain version for my ESL hotel students and am up to-my-eyes in lesson planning. Maybe leave this one for a quieter week.

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  • http://www.onestopenglish.com/support/ask-the-experts/grammar-questions/grammar-differences-between-could-and-can/146362.article?PageNo=1&SortOrder=dateadded&PageSize=50#comments

    You've identified the major ambiguities...

    "Both can and could (and other modals, especially may and might) are used to express various kinds of possibility, ability, permission and potential."
    "Could, of course, functions as the past tense of can, and like other past forms, it....can also indicate things like tentativeness, indirectness, deference and a wish not to impose."

    ...but your response should be split into four fundamental use cases:

    In the past tense, could indicates ability--not possibility--as the past tense of can; among the modal verbs, could is the only preterite regularly used as an ordinary past tense:
    • "In my youth, I could drink all night and be fully functional come morning,"
    Means , "In my youth, I was able to drink all night and be fully functional come morning."
    Further examples might include:
    • "I could leap the back gate, but I couldn't scale the garden wall."
    • " Steve Scott could run a four-minute mile at will."

    In the present tense, could expresses possibility (in a particular case)--but so does can (in an ongoing- or general situation):
    • "I could drink all night and be fully functional in the morning," would refer (in present tense) to tonight;
    • "I can drink all night and be fully functional in the morning" would refer to any night.
    Further examples might include:
    • "We could be looking at a costly citation here," versus "Speeders can expect heavy fines in Texas;"
    • "Recent downpours could cause this parking lot to flood," versus "Heavy rains can cause flooding in low-lying areas."

    As the conditional form of can, could is also commonly used in conditional sentences:
    • would + can = could
    Examples might include:
    • "If we had some eggs I could fix you an omelette;"
    • "I could wash the car if you'd paid the water bill."

    As non-deontic indicators of volitivity, can and could introduce increasing degrees of implied uncertainty when used to initiate a request:
    • "Can you open the window?" and
    • "Could you open the window?"
    Each means "Please open the window," with could implying the greater reluctance to impose.

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  • It depends. If y o u is stressed, it would take on a different meaning (Like can you because I can't). As a question I'd use "Could you or Would you be so kind as to...please"

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  • We use can when we expect the answer to be yes
    "Can you open the window?"

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  • yes, very helpful thanks.
    I also think that we often weaken the stress on "can" and add.. "If you like/if you want" to make it more hypothetical/tentative and more like "could".
    We can go to the cinema if you like, or just watch TV.

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