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I want to ask a question about the usage of an auxiliary verb "can."

I think the next sentence is grammatically incorrect.

This story can be true.

In contrast, I think the next sentence is correct.

This story could[might/may] be true.

Is my understanding right?

supplementary explanation

In a dictionary, I learned about "can" that expresses possibility (not ability or permission) as follows:

In positive sentences, "can" expresses general possibility, that is, possibility that a thing happens irregularly several times, and "can" isn't able to express specific possibility, that is, possibility that a thing happens specifically only once.

For example, the next sentence is not correct.

It can rain tomorrow.

In contrast, the following sentence is correct.

It may rain tomorrow.

(I think not only "may" but also "could"/"might" are correct.)

That's why I thought "This story can be true" is not correct. For possibility that this story is true is specific, that is, it happens specifically only once.

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    They're all grammatical. It's just that the can version is an incredibly unlikely utterance compared to the others, because semantically it wouldn't normally make sense in most contexts. It's just about possible to imagine someone who's not very good at maths saying 2 plus 2 could / might / may be 4 (I can certainly imagine The square root of 132365569809 could / might / may be an integer). But it's almost impossible to imagine a context where someone could validly say 2 plus 2 can be 4, even though it's syntactically fine.
    – FumbleFingers
    Commented May 9, 2021 at 13:46
  • I made a supplementary explanation. I'd appreciate it if you would read it.
    – Aya
    Commented May 11, 2021 at 4:22
  • I don't think "specifically only once" is meaningful here, but you're quite right that utterances like This story can be true and It can rain tomorrow are essentially "incorrect" (except in very unusual contexts that are hardly worth bothering about, since they will probably never arise). Note that whereas I can say to you Pick a card. It can be red or black (because there is still a choice in play), I CAN'T say Now let's talk about that card you just picked, which can be red or black. Because there's no longer a choice it has to be could / may / might be.
    – FumbleFingers
    Commented May 11, 2021 at 12:35

2 Answers 2

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Your understanding is incorrect. All of the following sentences are correct:

"This story can be true"

"This story could be true"

"This story might be true"

"This story may be true"

To me, can and could imply possibility, whereas may and might implies a slim possibility, (e.g. "it might, if all goes well"). Also, to me, can implies that certain conditions have to be true, for example "this story can be true if you make a small donation"

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    You mean "Your understanding is incorrect" because the OP claims that "This story can be true" is not good grammar.
    – James K
    Commented May 9, 2021 at 13:20
  • wait.. i completely missed the grammatically incorrect bit there sorry
    – flumperious
    Commented May 9, 2021 at 13:23
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"This story may/might be true" is simple and common. It expresses a possibility, or uncertainty. The speaker does not know if the story is true or not.

"This story can be true" is grammatically correct, but less common. We often use "can" when something is potential. When the speaker is able to do something. So this would be used when one is able to make the story true. That is possible in the following context.

Teacher: I want you to write an essay about your favourite story. This story can be a fairytale, or a modern story, or this story can be true. You can choose the story.

We also use "can" to express greater confidence than "might":

When my uncle told me that bigfoot had taken his picnic, I didn't believe him. But when I saw the footprints and saw the photos my uncle had taken I realised that his story can be true!

It is less common than "might be true" but it is grammatically correct and sometimes idiomatic.

However, usually "might be true", "may be true", "is likely to be true", "is probably true" are better ways to express a range of doubt

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  • In your "teacher" example, can essentially modifies choose (or equivalent) even if that verb isn't explicitly specified. If the teacher had started by saying We're going to study a short story written by X, it wouldn't normally be possible to continue with This story can be true, even if the teacher doesn't know whether it's true or not. He could certainly say It could / might / may be true to reflect uncertainty in that situation - but not can, which reflects ability to change / make a choice (meaningless where "objective truth" inherently applies).
    – FumbleFingers
    Commented May 11, 2021 at 11:31
  • IMO, the "can" In "I realized that his story can be true!" sounds a bit awkward. "Could" sounds a bit more natural.
    – Brian
    Commented May 12, 2021 at 20:29
  • Does "can" express greater confidence than "might"? I first heard such an explanation. Also, I learned "could" expresses less great confidence than "might". You mean "can" and "could" express almost the same meaning as "must"? For example, does the next sentence have almost the same meaning even if must is replaced with can / could? "He must be clever since he solved the difficult problem."
    – Aya
    Commented May 13, 2021 at 15:01
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    (from wikipedia) I can speak English means "I am able to speak English" or "I know how to speak English." You can smoke here means "you may (are permitted to) smoke here" ..... There can be strong rivalry between siblings means that such rivalry is possible......
    – James K
    Commented May 13, 2021 at 17:18

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