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Is there any difference between these two statements. If yes could you tell me when to use them.

  1. I have to do that
  2. I will have to do that
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    I think this is Off Topic General Reference for ELU, but it would have been okay on English Language Learners FWIW, in most contexts OP's two alternatives are semantically equivalent. Usually you say you have to do something when you're not doing it now, but will of necessity do it at some point in the future. We can't "inflect" the specific infinitive verb (do, or whatever) to indicate that future tense, so we just do it to the have to component instead.
    – FumbleFingers
    Commented Oct 10, 2013 at 21:09

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The difference is in the verb tense of the sentence. I think the difference will be more apparent if I modify your example slightly.

"I need to purchase gasoline."

"I will need to purchase gasoline."

The first statement indicates that this need is occurring at this moment in time. The second statement indicates that this need will occur at a time in the future.

The second sentence is an example of simple future tense, whereas the first sentence is an example of the simple present tense.

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    Yes – good switch. The 'have to' construction was complicating the root issue.
    – Edwin Ashworth
    Commented Oct 10, 2013 at 21:03
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The difference is that the idiom have to (always pronounced /hæftə/, never /hævtə/)
is in the present tense in sentence (1),
but is an infinitive in sentence (2).

You can't tell this from the sentences,
because both are spelled -- and pronounced -- the same way,
but you can tell if you change the subject from I to Bill,
because the present verb changes to has, but not the infinitive:

  • Bill has to do that.
  • Bill will have to do that.

Now some will tell you that this is the "Future Tense" in English.
They're wrong. It's just a normal use of the modal auxiliary verb will,
which must be followed, like all other modal auxiliary verbs
(i.e, can, may, must, shall, should, might, could, would),
by the infinitive form of the next verb,
which in this case is the idiomatic modal paraphrase hafta
(or spell it have to, if you prefer).

It's not any more "Future" than sentence (1),
which is after all, about the future,
nor is it any more "Future" than

  • I'm gonna hafta do that (pronounced [ãmə̃nə̃'hæftə'duðæt],
    -- or spell it I'm going to have to do that, if you prefer)

All mean the same, and all are acceptable.

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    The idiom have to may always be pronounced /hæftə/ and never /hævtə/ in the States, John, but I'd say that's far from true in the UK. Even the schwa isn't always used.
    – Edwin Ashworth
    Commented Oct 11, 2013 at 10:38
  • Fast speech rules vary widely. I can only speak for American English, where final vowel centralization is largely a function of speed and familiarity.
    – John Lawler
    Commented Mar 17, 2015 at 1:58
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    When my wife and I visited the Southwest, we had no problems at all with the language, either understanding or jar-factor. The same can not be said for the time we met a pleasant lad from Sunderland a few years ago.
    – Edwin Ashworth
    Commented Mar 17, 2015 at 9:49

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