What's the difference between "I have a plan" and "I do have a plan"?
I wanted to know why some affirmations have the "do", I thought it was only necessary to use it in negative and interrogative sentences. (http://translate.google.com.br/)

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If someone says:

"Why don't you have a plan for a situation like this?"

You can say...

"I DO have a plan!"

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I'm not knowledgeable on grammar but, "I do have a plan" would be said if who you were talking to didn't believe you had a real plan.  Adding the "do" adds emphasis, but more importantly it serves to convince others who might be skeptical.

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The one with "do" has more emphasis that you have a plan. "Do" gives more sureness, impact and emphasis to the sentence.

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Emphasis.