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I have seen some topics related to "Did you know?" and "Do you know?" questions, and when we use them.

My question is not when we use "Did you know?", But it is Why, Why do we need to use past form with this question?

The affirmative statement is;

  • You know ...
  • I know that you know how to make more money.
  • You know I needed many things ...

So, the question is:

  • Do you know ...
  • Do you know how to make more money?
  • Do you know I needed many things ...?

I think the question should be use present simple to ask about things that others know now.

So, What is the reason to use "Did you know"? Is it idiom?

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    I think it's partly idiomatic, but also because we really are discussing a past state of knowledge. For example, I would say "Did you know that rats cannot vomit?" because you do now, because I just told you that fact. It wouldn't make sense to say "Do you know?" because as of now, you do know. – stangdon Feb 23 '17 at 11:19
  • It is short for "did you already know (information x).", typically "do you know" is a request for (information.x) – Peter Morris Feb 23 '17 at 23:00
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There are two uses for using the past in your example

Did you know

Did you know that February has 29 days every four years?
Do you know that February has 29 days every four years?

The first question is more wondering if something is common knowledge for most people, the second is just a question to the person being spoken to.

P1: I didn't know how often the train comes by.
      Did you know that the train comes by every hour on the hour?
P2: Why yes I did, it's in the schedule.

It is also used to shift the timeframe of the question to the past and is meant to gain the understanding or viewpoint of the questionee at that point in time

You brought an umbrella, did you know it was going to rain?
Did you know it was going to rain before you left home?

would be asked after it has started raining, since

You brought an umbrella, do you know if it is going to rain?

would not make any sense at that point.

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