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Use of “do” in affirmative statements

Why do people sometimes use the words "do" or "does" in affirmative sentences?

For example:

A: We know a guy!

B: We do know a guy!

Why doesn't B just say: "Yes, we know a guy"?

And in general, why the use of "do" in affirmative sentences?

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marked as duplicate by RegDwigнt Aug 15 '12 at 22:30

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

up vote 4 down vote accepted

Do in affirmative sentences is used to put an emphasis on the positive fact.

Person A: I've heard you don't have the piano anymore.
Person B: Oh, that's not true! I do have the piano!

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On a tangent, what is that "do" called? Is there a word for it? – Daniel Sep 6 '11 at 18:58
@dr65--It's generally do-support, but I'm not sure what "do" really is. – simchona Sep 6 '11 at 19:21
@drm65, a modal verb? – Thursagen Sep 6 '11 at 21:24

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