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I heard some sentences with this form from English speakers: "what I do know is that..." or " We do share the same..." The question is: what is the applicant of do in these sentences? Why they use it before main verbs?

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It's one of the many uses of "do-support", which is worth researching generally.

In this case it either adds emphasis, or contrasts with a previous negative:

I don't know if the case you are thinking of contrasted with a previous negative, but I do know this sentence does.

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