I was told that this is wrong: What day will be tomorrow?
Yes, that is wrong because, if you treat "tomorrow" as a noun, and therefore as the subject of the question, you should say:
- What day will tomorrow be? (not idiomatic)
In the question:
- What day will it be tomorrow?
"tomorrow" is an adverb of time, and "it" is the subject.
DO you agree? If yes, why is this correct?
What man will come tomorrow?
In the question above, "what man" is the subject of the sentence.
A: What man will come tomorrow?
B: The man you know will come tomorrow.
Instead, in the previous question "what day" is the subject complement, since you don't answer: *Wednesday will be tomorrow but Tomorrow (it) will be Wednesday.