As snailboat pointed out in a comment on your question, the premise of this question is mistaken. "What will you do tomorrow?" is grammatical, acceptable and idiomatic in certain circumstances. For example:
Anna: Oh no! My running shoes are ruined! Now I won't be able to go for my usual evening run tomorrow.
Joel: Oh dear! What will you do tomorrow evening instead?
The difference between "What will you do tomorrow?" and "What are you doing tomorrow?" is not a question of grammaticality - both are grammatical - or even one of how idiomatic the sentence appears. The difference is what the question is asking:
"What are you doing tomorrow?" is a question that asks the listener what plans they have already made for tomorrow. It is passive, and is merely asking for information from the listener.
"What will you do tomorrow?" is a question that asks the listener to make a decision about what to do tomorrow. This question is more active. It asks the listener to create a plan and then tell the questioner what the newly formulated plan is.
Consequently, after some event that forces the listener to change their plans for tomorrow, "What will you do tomorrow [now that your previous plan is no longer valid]?" is more idiomatic than "What are you doing tomorrow [now that your previous plan is no longer valid]?", because there has not been time to produce a new plan, and the listener must therefore create a new one, rather than merely recite the plan that they had already made.
In contrast, if the listener has indicated that they are busy tomorrow, "What are you doing tomorrow?" is more appropriate, since the implication is that the listener has already made plans, and the questioner is inquiring what those plans might be.
There is also a small difference in subtext to the two questions as well. "What are you doing tomorrow?" is more passive, but also includes a possible subtext of asking whether the questioner can join in the activity. "What will you do tomorrow?" on the other hand contains a possible subtext of asking what the listener will do without the speaker, and can thus be construed as more cold.
Both are correct, but most proffered one is
I have time tomorrow.
This means, I have time definitely. On the other hand It is expressed with a confirmation.
- [Subject] = I
- [Verb] = have
- [Object] = time
- [Predicate] = tomorrow
I will have time tomorrow.
This means, I am not sure that I have time or not. On the other hand I have time sometimes.
- [Subject] = I
- [Auxiliary Verb] = will
- [Verb] = have
- [Object] = time
- [Predicate] = tomorrow
Best Answer
ADDENDUM to Tyler James Young's comment and answers by user3169, nicael, and dantiston
(Please don't upvote this: it doesn't address the main question, the use of futurive will.)
These answers tell you that the there BE construction (the ‘existential’ construction) is not ordinarily used for statements of this sort.
The reasons are complicated—the Cambridge Grammar of the English Language, for instance, devotes five and a half pages to ‘pragmatic constraints’ on the existential construction, and CGEL is just a summary!
But a good rule of thumb is that the primary purpose of this construction is to present its complement, what follows the There BE, as new information—it announces the existence or occurrence of something which the speaker presumes the hearers don’t know about. For example:
This assumes that the hearers don’t know about this match.
Note that the speaker uses a match; the indefinite article also suggests new information. But the match and Liverpool’s match use definite determiners; these mark old information, things that the hearers already know about, so they don’t suit the existential construction very well.
However, there are some circumstances where the existential construction does accept definite complements. For instance:
In this case the match information is ‘hearer-old’—I know you already know about it, so I use the. But I believe you have overlooked it, or forgotten it, so I can use the existential construction because it is ‘discourse-new’—I am bringing it into the conversation for the first time.