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I think both modals 'could' and 'can' denote possibility but the ways they do is different. I can imagine some situations they are interchangeable but I think most of their application cases are different.

Could

He could be the one who stole my candy a few minutes ago.

Can

He can be the one who stole my candy a few minutes ago? -- to me, awkward, if not wrong.
You can submit your homework next week.-- this seems to be the right use of this possibility notion?

What do you think of what I said? Can you help me to gain more clear ideas on this?
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Modals such as can and could are useful, as you say, for denoting possibility or degree of certainty/uncertainty. And you are right -- the degree of such possibility is different for can and could, and "He can be the one..." is awkward (though not wrong grammatically -- it just doesn't convey the appropriate level of uncertainty).

Modals have a second, slightly different set of meanings in addition to the one you identified. They can be used to denote permission or ability, rather than possibility/uncertainty. (Swan has a good explanation of this, but I don't have mine handy. Anyone else?)

--sandy
AnonymousI think both modals 'could' and 'can' denote possibility
Hmmm. No. Not 'can', particularly. may, might, and could are the possibility modals.
He can be the one ... is more like giving permission. Actually, it's like assigning a role in the guise of giving permission.

Let's play doctor. You can be the patient, and I can be the doctor.

You can ... is exactly a case of giving permission.
You can submit your homework next week; you don't have to submit it this week.
CJ
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Thank you. Why does the Englishpage.com's tutorial on modal verbs say about the use of 'can' this way? This is part of their explanation on the use of the moddal verb 'can'. Dp you still hold on to your position that 'can' can't denote possibility (if I interpreted your reponse correctly).

Can


"Can" is one of the most commonly used modal verbs in English. It can be used to express ability or opportunity, to request or offer permission, and to show possibility or impossibility.

Examples:
  • I can ride a horse. ability
  • We can stay with my brother when we are in Paris. opportunity
  • She cannot stay out after 10 PM. permission
  • Can you hand me the stapler? request
  • Any child can grow up to be president. possibility
  • Using "Can" in Present, Past, and Future

    Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "can" behaves in different contexts.
CalifJim
AnonymousI think both modals 'could' and 'can' denote possibility
Hmmm. No. Not 'can', particularly. may, might, and could are the possibility modals.

He can be the one ... is more like giving permission. Actually, it's like assigning a role in the guise of giving permission.

I'm not saying that I disagree, Jim, but it's my impression that English is messy enough for nearly any defective auxiliary to have nearly any meaning in the right circumstances. There's a notice on the doors of all subway trains in London: Obstructing the doors causes delay and can be dangerous.

CB
(I edited this because my examples didn't match the point I was trying to make! -s)

So, am I the only one who thinks it's more than okay (though darned confusing to ESL students) to use "could" as a past form of "can" (when an attempt was made and succeeded)? CJ, you say it's correct only in the negative, but I would say it's common usage in the positive also:

I practiced and practiced that piece until at least last I could play it without errors. (You say that is incorrect and must be "i was able to"?)

My son tried to lift the weight in spite of my telling him it was too heavy, and it turned out that he could lift it! (Incorrect? Must be "he was able to lift it"?)
-sandy

P.S. CB, thank you for introducing to me the term "defective auxiliary" -- I have a student who will enjoy that terminology for these darned modals!
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OrlandoEnglishTutorP.S. CB, thank you for introducing to me the term "defective auxiliary" -- I have a student who will enjoy that terminology for these darned modals!

My pleasure! Emotion: smile It's not my term originally. I think I learned it when I read George O. Curme'sSyntax.

CB
AnonymousDp you still hold on to your position that 'can' can't denote possibility (if I interpreted your reponse correctly).
Here's the correct interpretation:
Not 'can', particularly.
That is, 'can' is not the central example of a modal expressing possibility. may, might, and could are the more "central" choices, the more "typical" examples.
I'll probably have more to say about this in a later post.
CJ
AnonymousExamples:
  • I can ride a horse. ability
  • We can stay with my brother when we are in Paris. opportunity
  • She cannot stay out after 10 PM. permission
  • Can you hand me the stapler? request
  • Any child can grow up to be president. possibility
I think the words in red are the culprits. A list like this presumes that we all know and agree upon what these categorial terms are, and that there is no overlap. That is, it assumes that even if there is overlap, we will interpret each sentence in the reading desired by the author -- not with any other possible meaning that might apply. Such lists are not totally useless, but nearly so. We can just as easily list some of these items thus:
I can ride a horse. knowledge (I know how to ride a horse.)

We can stay with my brother when we are in Paris. permission (We have permission to stay ...)

She cannot stay out after 10 PM. obligation (She is obliged to be home by 10 PM.)

Can you hand me the stapler? capability (Are you capable of handing me ...?)

Any child can grow up to be president. potential (Every child has the potential to become president.)
If you had seen this list instead, the question of whether can signifies possibility might not even have come up, because this new list doesn't have "possibility" as a label for any of these uses of can.

CJ
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