I'm an ESL teacher. A student asked me a question regarding this sentence.
"Don't phone Ann now. She'll be busy." The student asked me why we have to use "will" in the sentence. Why shouldn't the sentence just be: "Don't phone Ann now. She's busy." I told her sentence one is predicting what is and the second is reporting what is, but I feel like I short changed my student. Perhaps there's a more detailed explanation someone might have?

  • 6
    Your explanation is fine and leaves nothing to be desired. She will be busy when he calls her, which is in the future.
    – RegDwigнt
    May 18, 2014 at 17:43
  • 2
    @RegDwigнt I suspect the OP is aware that the explanation is correct, but would like a more detailed answer to give to the student. Basically it's prediction with an element of uncertainty/assumption (She'll be busy) versus certainty (she's busy).
    – Mari-Lou A
    May 18, 2014 at 17:55
  • In most contexts, the two versions will mean the same thing. In some specific contexts, one version might be preferable to the other--that is, it might have a sightly different meaning. Your explanation as to the difference between the two seems pretty good to me. If you feel like you've short-changed your student, you could perhaps create two situations where one version of your examples would be preferable to the other--actually, that would be a good exercise for your students to do. :)
    – F.E.
    May 18, 2014 at 18:52

2 Answers 2


Will is a modal auxiliary verb, and, like all the English modal auxiliary verbs

can, could, shall, should, may, might, will, would, must, and sometimes need and dare

will has two senses, called its deontic /de'yantɪk/ and its epistemic /ɛpə'stɛmɪk/ senses:
(Sorry; I don't make up these names, honest. They're standard in the literature.)

  1. The deontic sense of will means that the subject is willing to do something.
    This is the only sense of will that occurs in hypothetical clauses with if, for instance.
    E.g, If he will hand in the homework, I will grade it means 'if he is willing to do it',
    with an invited inference that he has previously refused to do it. The will in the second clause can either be deontic (I am willing to correct it) or an epistemic prediction.

  2. The epistemic sense of will means the speaker's personal judgement of
    (in this case) the subject's probability of being busy at some time in the future.
    This is the sense of will that is often called "the future tense" in Latinate grammar textbooks.

It is the case, however, that all modal auxiliary verbs can, and usually do, refer to the future;
since there is no future inflectional tense in English, modals are often used for the purpose,
and are often arrayed to express differences in the provenance of the judgement.

Though I have always wondered how will (and occasionally shall, though not in America)
got singled out for accolade of "Future Tense", ignoring all the other modals.
My best theory so far is that it's because epistemic will has so little other meaning,
and because it makes a nice contrast with the other future tense, going to.

As pointed out in the other answers, in this context, in this sentence

Don't phone Ann now; she'll be busy.

the use of she'll be, instead of she's, expresses the speaker's best guess,
whereas the use of she's expresses the speaker's certainty.

And other modals could express other modalities, using the same structure --
many of them are ambiguous between epistemic and deontic senses:

  • Don't phone Ann now; she could be busy.
  • Don't phone Ann now; she should be busy.
  • Don't phone Ann now; she may be busy.
  • Don't phone Ann now; she might be busy.
  • Don't phone Ann now; she will be busy.
  • Don't phone Ann now; she would be busy.
  • Don't phone Ann now; she must be busy.

(I ignore periphrastic modals like hafta, gotta, oughta, etc. here. Life is short.)

  • 1
    I just wanted to say how great it is that you take the time to answer my question with this much detail. Really great and considerate and super and a lot of other nice words your way. Thanks.
    – user76539
    May 21, 2014 at 18:09
  • You're welcome. I started my career teaching ESL for 6 years; I understand how difficult it is to get things straight and explain them clearly. May 21, 2014 at 18:10
  • Are you sure that the "will" in "If he will hand in the homework" is an instance of the deontic use of "will"? I'm asking this because the deontic use has the meaning of some sort of duty or obligation imposed by the speaker, which does not obtain in your if-clause example.
    – JK2
    Sep 23, 2014 at 3:21
  • The deontic use of will can have those meanings, especially if used in a command: "You will clean your room!" But normally it means 'want, desire, be willing to'. Any duty or obligation is self-imposed by the will. Sep 23, 2014 at 14:13

There are pragmatic implications beyond the obvious.

"Don't phone Ann now. She's busy."

This is a statement of a known existing state (eg I know she's got three essays to complete in the next four days, or when I left her 30 minutes ago she had to pick her two kids up, feed them and get them to the Scouts).

"Don't phone Ann now. She'll be busy."

This is a statement of a very probable state, though the probability factor is less than the certainty expressed in the alternative. She may have three essays to complete in the next ten days, say. She needs time, though perhaps at this instant she's just having tea.

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