1

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?

  • 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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    @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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    I'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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    It 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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    Oh grammar book, grammar book. Whyfore art thou useful? – Andrew Apr 3 '17 at 18:13
3

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.

(BBC Learning English)

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