Can anyone who knows the language well or is a teacher please tell me what the difference between cannot and will not be able to in the following sentence is?

Here's the sentence:

  • John, I am very ill. I am sorry to say that I cannot/will not be able to join you in the meeting tomorrow.

My question is that as I have used tomorrow in that sentence, which means future, Shouldn't I use will not be able to?

".............that I cannot/will not be able to join you in the meeting tomorrow."

  • We can use the present to refer to a future. I {cannot | am unable to} attend the conference next week.
    – TimR
    Commented Jul 23, 2018 at 18:50

3 Answers 3


There is no functional difference here, so you can use either cannot or will not be able to.

The reason you can use cannot (present tense) to apply to a future event is because the reason is: (a) true in the present, and (b) known to apply to tomorrow as well (that's the meaning as written, at least).

Your reasoning appears to be that you are describing a future action alone, in which case the future tense would be necessary. But a native speaker would recognize that you are describing the current state of your schedule, which happens to relate to a future event.


First, as a matter of fact, most of the time future reference does not involve the use of will.
Will is just one more modal auxiliary verb, not a special tense. Any modal auxiliary can refer
to the future. So you needn't worry whether you need will.

Second, while either phrasing will work as a polite negative response to a meeting request,
they don't have the same syntactic structure and they don't mean quite the same things.

  • I cannot join you in the meeting tomorrow.

simply means that you won't be there and pleads inability, without specifying why you are unable.
It might be that you're forbidden, or that you're in the hospital, or that you're in Capetown, or that something else came up.


  • I will not be able to join you in the meeting tomorrow.

specifically refers to the future (by using will), and thus suggests that the day after tomorrow might be possible -- or maybe even later today -- but not tomorrow. But it also gives no information about the reason, aside from suggesting that it might be temporary.

  • First you say it isn't future, then you say it refers specifically to the future. So, I dunno.
    – Lambie
    Commented Jul 23, 2018 at 20:12
  • It's not future tense. There is no future tense in English. However, many teachers have been told that will is "future tense" and they dutifully repeat it, though it's never been true. Will can refer to the future (most modals do, after all -- permissions and obligations mostly happen in the future). But that doesn't make them future tense; there are only two tenses in English -- present and past. Everything else is special constructions with auxiliary verbs and other markers like complementizers and prepositions, and there are hundreds of them, and they're not tenses. Commented Jul 23, 2018 at 20:19
  • That is an opinion. For me, there is future intention: I will go when I can. I have taught English for 35 years. I don't call it future tense. But it governs future action. I call it the "uses of will". But it does convey an action that will occur in the future, even just around the corner.
    – Lambie
    Commented Jul 23, 2018 at 20:36
  • No, future intention is fine, and there's all sorts of ways we refer to it. The "future tense" sense of will is its normal epistemic meaning of 'predicted; expected'. But there is also its normal deontic sense of be willing (to), as in this answer. Commented Jul 23, 2018 at 21:16
  • This present/past verb tense position is your opinion. Then, you would have to say that French, Spanish, Portuguese and Italian have no future tense either....One can in certain linguistic situations use your position, I agree. I do not think it's pedagogically useful myself. I also think that to-infinitives don't exist. I think to + verb is silly. But I use it for teaching purposes.
    – Lambie
    Commented Jul 23, 2018 at 21:32

Meanings occur in context.

I am ill so I cannot join you tomorrow or will not be able to join you tomorrow. [either can be used, cannot can refer to a time in the future].

Question: Can you attend the meeting tomorrow? Answer: Yes, I can [my schedule allows it or health allow it.] but I'm afraid I won't. [will of intention] I never attend meetings at your office.

Question: I know you are ill. Will you be able to join us tomorrow? [Does your physical state or other state allow you to join us? Specifically. ] Answer: I will not be able to join you tomorrow. [am physically unable to do so then]

Question: Will you open the window?

Answer: Yes, I will. [will used in asking someone to do something].

Question: Will you attend the meeting tomorrow?

I would go if I could, but I simply cannot go because I am unable to but I will go in the future, I promise.

[parse: conditional, conditional of can, negative reference to a future and future intention and physical incapacity.] The cannot here is: it is not possible for me to go due to a physical or other situation that stops me from going.

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