Why people say "Please tell me what you need ." , instead of "Please tell me what do you need." ? Why I can't add this word "do" in this sentence? Could anyone explain it for me by grammar?

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Answer

The auxiliary verb 'do' is not used because this is not a question. It is an imperative sentence.

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It's an indirect question. You don't use "do/does/did" in indirect questions. An indirect question could also be embedded within another question and you still wouldn't use a form of "do":

Why won't you tell me what you need?

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You can use it, but you need a comma or a some kind of a pause:

"please tell me, what do you need?"

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Hello pretty girl

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