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In grammar books, it is written that you can use "cannot" or "can't" for negative deduction like:

'Susy cannot stay at her mother's house because she is in hospital.'

I know that this works for situations in the present but what about negative deductions referring to the future like:

'Tony cannot come to his sister's bithday party tonight because he is in hospital.'

meaning that I conclude from Tony's situation that it is not possible or true he will be at his sister's birthday party tonight or would only this sentence be correct?

'Tony will not be able to come to his sister's birthday party because he is in hospital.'

What about in conditional sentences in which the main clause refers to the future like:

'If my son ever breaks the table, we cannot use it.'

or is there only one correct way of putting this sentence:

'If my son ever breaks the table, we will not be able to use it.'

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The trick is that "can't" always needs a true statement (at the time of speaking) that is happening now, will happen in future, has happened just now, or happened in the past:

  • John can't come tonight because he is in another city.
  • John can't come tonight because he will be in another city.
  • John can't come tonight because yesterday he went to another city.
  • John can't come tonight because he is going to another city

All of these are valid and well, common, in informal English especially.

But when the statement is abstract or unpredictable then "can't" simply doesn't work:

  • John can't come tonight if he goes to another city.
  • John can't come tonight unless he doesn't go to another city. (sounds a bit silly, I know)

The point is - all the "if", and "unless" clauses are unpredictable. Only "be able to" really works in this case.

  • John won't be able to come tonight if he goes to another city.
  • John won't be able to come tonight unless he doesn't go to another city.
  • I know this may sound silly but it's like that in speech. At least it's what I hear. – SovereignSun Nov 2 '17 at 17:36
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For the future case, either sounds fine to me. In fact, I think "can't" would be much more likely to be used, certainly in spoken language, since "will not be able to" is quite long in comparison.

The table case, though, doesn't sound right at all. "We won't be able to..." is much better.

I'd think about using contractions especially in the cases of "cannot" and "will not" as the uncontracted words sound very clumsy. (There's a reason why TV shoes like Star Trek seldom have their aliens use contractions! When Data's brother Lore uses contractions, we can see how much more natural he sounds.)

By the way, I'd say "... she is in THE hospital."

  • 3
    "In hospital" is correct in British English. American English would always use "the hospital". – PMV Jun 5 '17 at 19:03

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