Context: My friend has been using computer a lot recently, I want to warn him

  1. Spending a lot of time using computer can lead to eye disorders.

  2. Spending a lot of time using computer could lead to eye disorders.

From what I learned, sentence (1) the speaker expresses a general truth, and he/she is quite sure about this this truth. Sentence (2) shows that the speaker isn't sure about this fact.

My English-speaking friends tell me that when using the two sentences, native speakers don't notice the difference between them. They use them interchangeably with the same meaning. Is that right?

  • 3
    Does this answer your question? What is the difference between can and could in 'Can/could you please explain this to me?'
    – Lambie
    Commented Oct 22, 2023 at 14:59
  • 1
    @Lambie the posts in this link is so general, and it doesn't have the same context as mine. Could you please help me with this specific context.?
    – LE123
    Commented Oct 22, 2023 at 15:03
  • 1
    can= is possible, could=is possible but less likely than can. But for 1, I would use "may lead to". "can" is often used for "may", be allowed to. While could is more of a possibility but not definite.
    – Lambie
    Commented Oct 22, 2023 at 15:08
  • 3
    [using a computer, by the way] Not just using computer.
    – Lambie
    Commented Oct 22, 2023 at 15:27
  • My main quibble with the closure is that while the duplicate does address the difference between the two, it doesn't seem to address the assertion in the final paragraph about whether native English speakers actually use them differently.
    – Catija
    Commented Oct 26, 2023 at 4:02

3 Answers 3


The two sentences are both common and natural ways to warn someone of the dangers of using the computer too much, but they arrive at their meaning in different ways, and have slightly different nuances.

Sentence 1 expresses an authoritative fact, and doesn't directly express anything about your friend at all. Your friend has to infer that you are warning them about how their use of computers may affect their eyes. With no context, this sentence doesn't imply any concern. This could be said to make someone feel bad. In the context of saying it to your friend, they'll definitely understand that you are warning them out of concern.

Sentence 2 directly expresses concern for your friend's eyes. It does not imply any authoritative knowledge, so it could be something you heard, or something you've always believed and don't know why exactly.

Again, the meanings are so similar it's probably not worth worrying about, but this info is here for completeness, and in case you really do care about these subtle differences.

  • 2
    This is the right answer. The two may be close in this particular example, but they do not mean the same thing, and I would expect a native speaker to be able to pick up on the difference. In addition to not implying any authoritative knowledge, could also entails less certainty: it is possible that, in theory, an eye disorder will be the outcome; but in practice, it’s at least equally possible that it won’t. With can, the possibility is considered real and tangible, both hypothetically and in practice. Commented Oct 23, 2023 at 10:48

Your friend is correct. Most native English speakers would see the two sentences as having the same meaning. Perhaps some language experts make a distinction between "can" and "could" in such a context, but most ordinary users of the language do not.

  • 1
    Two words express two different meanings, so I don’t understand why native speakers treat them the same in practice
    – LE123
    Commented Oct 23, 2023 at 6:59
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    @LEHANH the different meaning is very nuanced. I think the point is that even if they are treated as the meaning what they actually are it wouldn't change the outcome at all. It doesn't really matter if "Some people think it causes eye disorders" or "Most people think it causes eye disorders". Both will be received as a general warning and whether you do something with that would probably be the same in both scenarios. The thing is also, people tell things all the time as if they are general knowledge. And it's also actually very subjective what is considered general knowledge.
    – Ivo
    Commented Oct 23, 2023 at 7:58
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    It may be that I’m a “language expert”, but I would absolutely make a distinction between the two, and I cannot imagine that most native speakers wouldn’t. Can is more likely than could. It’s the difference between an empirically known possibility and a hypothetical possibility. Whether that distinction is significant in this particular context is a different matter, but it is definitely there. Commented Oct 23, 2023 at 10:51
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    @JanusBahsJacquet: Agreed; I (as a native speaker) would definitely make a distinction between the two.
    – psmears
    Commented Oct 23, 2023 at 14:36
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    @JanusBahsJacquet I agree with you, but I also think this answer is correct. Unless I knew the speaker was being precise in their language, assuming this distinction is risky. If the distinction is important, it would be wise to ask for clarification.
    – JimmyJames
    Commented Oct 23, 2023 at 19:33

In a case like this, the practical difference between the two words is social, not a matter of definition. I've never heard of "can" holding more authority than "could"; certainly never seen any official definition stating this to be true.

They are both slang terms replacing another word. They mean the same thing and every native English speaker will understand that upon hearing the sentence, though the person may prefer one over the other.

Both words fill in for a few other words. The definition of "can" changes depending on which word it is replacing.

"Can" filling in for "may":

  • Sitting at the computer can hurt your eyes. / Sitting at the computer may hurt your eyes.

"Could" is used to replace "may" as well.

  • Sitting at the computer could hurt your eyes. / Sitting at the computer may hurt your eyes.

"Can"/"Could" filling in for "will":

  • I can/could do that. / I will do that.
  • Can/could you help / Will you help?

Unless you're speaking / writing formally, it's unlikely that which word you use really matters, though as others have mentioned, you're more likely to be told "yes" to your question when you use the word which the other person prefers.

When in doubt, I stay away from slang. I might sound a little snobby, but it provides for clearer communication and avoids confusion like this. You can even turn to science if you don't want the personal responsibility of being accurate:

"Studies have shown that spending too much time at the computer leads to eye disorders."

Hope this helps clear things up! :)

  • 1
    Slang terms?? No way. Sorry.
    – Lambie
    Commented Oct 24, 2023 at 0:17
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    Re ""can" ... "could" ... They are both slang terms": Extraordinary claims require extraordinary evidence. Modal Verbs | MUST CAN WOULD SHOULD MIGHT WILL COULD SHALL MAY Commented Oct 24, 2023 at 0:18
  • Sorry if I offended anyone, I know "can" is from old English, likely from the German "kann", not an informal derivation. I was trying to keep this simple-ish as I didn't think this was the place to get too technical. I'm just trying to answer the question in a practical and informative way. Call it slang or don't, but if you find yourself in court, I highly recommend asking "May I approach the bench, your honour?" instead of "Yo, can we talk real quick?"
    – SomeGuy69
    Commented Oct 24, 2023 at 0:28
  • 1
    Using can in the sense ‘is allowed to’ is widely considered to be informal, and using may would be less marked (though some would instead perceive can as neutral and may as formal or old-fashioned). But it’s absolutely not slang by any stretch of the definition of that word. It’s also not a “filling in” for anything, just a core meaning of the word – and of course, it’s not the meaning in the sentence in the question. And there is a world of difference between ‘I can do that’ and ‘I will do that’ – those two aren’t even close in meaning. Commented Oct 24, 2023 at 21:40
  • (And can is not derived from German kann. They are related, but both are derived from the same Proto-Germanic source, not one from the other.) Commented Oct 24, 2023 at 21:41

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