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As explained in this question on the English site, there is a distinction in English between saying "How did you know?" and "How do you know?"

When someone makes an assertion, the distinction between "how did you know" and "how do you know" seems to be that "how did you know" implies that the person in question is correct in their assertion. "How do you know" is normally an inquiry into the person's credentials, and often expresses that the assertion is incorrect and/or ungrounded.

How would you say these expressions in Japanese, and is the distinction made by tense, as it is in English? That is, are they "どうして知っていましたか。" and "どうして知っていますか。"?

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Related: “How should I choose between 知る and わかる?” by Derek Schaab. – Tsuyoshi Ito Feb 24 '13 at 19:53
up vote 7 down vote accepted

I think that どうしてわかったのですか and どうしてわかるのですか correspond to “How did you know” and “How do you know” in the way you described.

どうして知っていましたか and どうして知っていますか do not sound right, but I am not sure why.

Unlike どうしてわかるのですか, どうして知っているのですか (Why do you have that knowledge?) implies that the assertion is correct. どうして知っていたのですか (Why did you have that knowledge?) also implies that the assertion is correct, but in this case, the assertion was made in the past.

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Just what I was looking for. Thank you! – Ataraxia Feb 24 '13 at 20:14

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