Which of the following is correct?
Your grammar is so good. Why do you need a grammar book for?
or
Your grammar is so good. What do you need a grammar book for?
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3@user178049 - You could, but what would you want to rephrase it like that for? – J.R.♦ Apr 3 '17 at 14:49
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2@J.R. I want to show that what actually refers to the complement of the preposition. And I don't think why makes sense if it's used like that. – user178049 Apr 3 '17 at 14:53
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1I'm voting to close this question as off-topic because the OP explains how they want the question answered, and their suggested approach is probably not the best way of explaining the difference. – JavaLatte Apr 3 '17 at 16:11
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1It might be useful to compare 1: What did you ask? and 2: What did you ask for? The first one simply asks you to clarify/repeat whatever your request was (maybe you wanted to know if the moon is made of green cheese, or whether someone would marry you). But the second one is ambiguous - it could mean the speaker wants to know why you asked a question, or it could mean he wants to know what specific thing you requested be given to you (by implication, something "real", as opposed to simply an answer to a question). But note that Why did you ask for? is simply ungrammatical. – FumbleFingers Reinstate Monica Apr 3 '17 at 18:02
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1Oh grammar book, grammar book. Whyfore art thou useful? – Andrew Apr 3 '17 at 18:13
What for is a set phrase meaning:
for what purpose or reason
(Merriam Webster). Basically it means the same thing as why.
You can either say:
What do you need a grammar book for?
or
Why do you need a grammar book?
Aside from JR's example in the comments, you can find more example sentences at:
LDOCE, Cambridge dictionary or Macmillan.
To make the usage of what ... for? clearer, you might ask yourself how would you answer such a question. For instance:
Why did you stop at Tom's office? (What did you stop at Tom's office for?)
You can reply: I stopped at his office to have a chat about our marketing strategy, but you can also say:
I stopped at his office for a chat about our marketing strategy.
