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I wonder if it is possible to use 'can' to signify possibility in the sentence below. I understand that 'could' or 'might' are more suitable here. The context is that we are expecting Jack to call today.

The phone is ringing. It can be Jack.

What is the difference in the meaning if we compare with usage of 'could' or 'might'.

  • I am no grammarian. It just sounds wrong. It can be your mother or it can be Jack, but I think you need *could'. It's something to do with tense. "Can it be Jack?" "Yes, it could." – WRX Feb 1 '17 at 22:11
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    Technically perhaps. But in context can implies a certainty that doesn't exist in your example. You have no way to know for sure before answering. Jack can call you, but there is no way to know who is now ringing the phone. – user3169 Feb 1 '17 at 22:31
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This is an interesting question because logically, it seems like "can" wouldn't necessarily break any grammar rules, but it sure sounds wrong. Willow Rex's comment on the question seems to have the right ring to it. I suspect that there is some context in which "can" would sound right. But the issue seems related to tense. This is my theory:

"Can" is about possibilities. There is a "trick question" used to teach principles of probability. Say you ask someone the question, "I just flipped a coin; what is the probability that it came up heads?" Most people would say 50%. But the answer is actually that it is either 0% or 100%. Probabilities are about the possibility of future events, not things that have already happened and the actual results are final.

That same principle applies to the phone call. A specific person has placed the call and that is the only person it can be, there are no other possibilities. We just don't know who it is. We will discover who it is when we answer the phone (a future event). That also parallels how we think about who it is. We don't think in terms of who theoretically originated the call, we think in terms of who will be at the other end when we pick up.

So knowing who it is is a future condition. "Can" is present tense and "could" is future tense, which makes "could" the appropriate word.

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Can is mostly synonymous with allowed to - not only in the sense of "someone is giving me permission to X" but also in the sense of "someone/something is giving me the ability to X."

The phone is ringing. It can be Jack.

This would only work if, for example, we are playing a game and the person writing this already knows the outcome, and wants to let the other players know that "Jack" is an allowed possibility.

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  • One nit -- for someone giving permission, may is preferred over can, and in the grammar of past decades, the correct choice. – RichF Mar 7 '17 at 11:57
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The phone is ringing. It can be Jack.

As far as you know, Jack is physically able to call you. It could also be anyone else able to dial a phone. No signal of likelihood, simply possibility.

The phone is ringing. It could be Jack.

Jack is not only able to physically call you, but you are expressing at least a slight likelihood of it being him.

The phone is ringing. It might be Jack.

This is sort of the opposite of the first case. You do not know if Jack is physically able to call you or not. But you think it likely that he call you around this time.

The difference in connotation between could and might probably varies as much with vocal intonation as dictionary meaning. Perhaps there are also regional variations. Let's compare three first-person cases to minimize unknown factors.

I can do that.
I can do that!

I have the physical ability to do that. As before, neutral on likelihood. The exclamation mark adds a degree of realization.

I could do that.
I could do that!

This becomes heavily context variant, and if spoken, intonation dependent. With the period, and no other context, it is almost as if you are silently adding, but I won't. With the exclamation mark, it again becomes a positive realization. I didn't think I could, but I can!

I might do that.
I might do that!

Again this would be context variant. An exclamation mark doesn't seem to change actual meaning that much. In either case you believe yourself to be (or will be) physically capable with some degree of likelihood. Stressing might in a certain way, though, can reverse it with an unstated, but I might not.

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  • From your explanation it seems that 'could' expresses stronger possibility than 'can' but Cambridge Dictionary says opposite: "Could in the present only expresses weak possibility. Can expresses strong possibility". Could you please clarify? – AlexD Feb 1 '17 at 23:47
  • It is also not clear how 'could' implies that Jack has ability to make a call but 'might' doesn't. – AlexD Feb 1 '17 at 23:52
  • @AlexD My point concerning can is that it is silent on the issue of likelihood. The fact that I can rob a bank says absolutely nothing about whether I will rob a bank. The opposite is not true. If I cannot rob the bank, then you know I will not rob the bank. One cannot presume that since the negative expresses unlikelihoodthat, the positive expresses likelihood. Could I rob a bank? Sure. But presume you know me well, and know that I've lied and shoplifted. You don't trust me. Would you tell a friend he can rob a bank, or he could rob a bank? (cont'd) – RichF Feb 2 '17 at 0:27
  • @AlexD Doesn't could here express more than physical capability, adding in an element of your knowledge of me as a person? Yeah, he could totally rob a f'n bank! // -- concerning might & could -- A fundamental meaning of could is "the possibility exists*, just as with can. The word might is more of a matter of expectation. Jack normally calls you Tuesdays at 7pm. It is 6:59 and the phone rings. It might be Jack. But he has been grounded and had his phone taken away. He cannot call you. But you don't know that, so from your, the speaker's perspective, hi might. – RichF Feb 2 '17 at 0:43
  • I don't agree with your "might" variation implying that we don't know if Jack can use a phone or not. I think "might" implies a lower confidence, whereas "could be" is for examples where you are expecting Jack to call and you are wondering if the current call is in fact him. – Peter Morris Mar 6 '17 at 17:30

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