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A native speaker said to me that this sentence is not correct with can, I have to use could.

The story can could be true.

He said that if a story can be true, it means that it is true. But if I'm not sure and I want to encode uncertanty I have to use could. But I think that any story is potentially true or not, it may turn out to be either way, so it actually can have this property to be true.

I also know that this sentence is correct:

The weather is nice but it can change later.

So if the weather can change when we are not sure that it will happen, so that the story can be true, when we are not sure that it is.

Can you settle this dispute?

  • Maybe there was some specific context that could render the sentence with can incorrect? I can't imagine either why "The story can be true" would be unacceptable. – stillenat Jun 23 '13 at 16:58
  • An interesting passage from Wikipedia: "The above negative forms (mustn't, needn't) are not usually used in the sense of confident assumption; here it is common to use can't to express confidence that something is not the case (as in It can't be here or, with the perfect, Sue can't have left)." (URL: en.wikipedia.org/wiki/English_modal_verbs#Must) – Graduate Jun 24 '13 at 2:04
  • I don't find the sentence to be entirely correct. The weather could change; weather can change. The difference is that if the weather could change, then it also might not, but weather in general has the ability to change. Can is an absolute statement of ability, could is a statement of a possibility. – BobRodes Jun 25 '13 at 4:53
  • @BobRodes: It is a good comment. So, the weather can change in general, but it could change in the next 4 hours. – Graduate Jun 25 '13 at 5:08
  • @Graduate: That's the idea exactly. – BobRodes Jun 29 '13 at 1:44
2

The story can be true.

This sentence is grammatical, but probably doesn't mean what you think. In the examples I can think of, it indicates a possibility that the the truth of the story can change:

Please tell me a story. The story can be true, or it can be made up.

or

You spend the rest of your days living leisurely on a beach. This story can be true, if you invest with Invesco(tm) today!

If you just heard a story and want to express your uncertainty about its truth, you could say

John told me he won the lottery. This story might be true.

It is equally common in my experience to use could here,

John told me he won the lottery. This story could be true.

A strict grammarian might say that this version actually implies some kind of conditional on the truth, like, "the story could be true, if he would turn in his lottery ticket," but this implication is usually ignored.

The weather is nice but it can change later.

This example is correct, but again could would be more common here.

Edit: According to a note in my Webster's New Collegiate Dictionary, "Can expresses primarily positive power of acting...". That is, can expresses a will to do something in addition to the ability or possibility to do it. This is why the word doesn't work well with subjects like "the story" or "the weather": These abstract subjects don't have a will of their own.

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The story can be true, or it can be made up.

Generally, sticklers will say that it is more correct to say "The story may be true, or it may be made up." "Can" is a bit more informal, for example, people will persist in saying "Can I have one of those?" in spite of being corrected to "May I have one of those?" by their teachers when young. The point is in this case we are talking about what is allowed, and that is "may".

Can in the formal sense denotes an absolute ability. Could denotes a conditional ability. So, "stories can be true" because some stories ARE true, but "this particular story could be true" because it might be true and might not be.

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The difference is in the time. This story either is or isn't true, but you don't know which. In such a case, you cannot use can. The weather is going to change in the future, and in this case you can use can.

Wrong:

*I don't know where John is. I suppose he can have gone to the supermarket.

Use could, may, or might.

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  • Is the sentence about John an example of wrong usage? – Graduate Jun 24 '13 at 23:27
  • Yes, it's supposed to be. I'll make it clearer. – Peter Shor Jun 24 '13 at 23:54

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