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In asking a question, which is preferred, "Did you know?" or "Do you know?"

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    They mean different things. did you know means "did you know at one point or another (but may or may not know anymore or don't remember)". do you know means "do you currently know". It's more likely that you'll want to ask do you know.
    – Dodgie
    Commented Dec 1, 2013 at 0:45
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    This question needs more context in order for it to remain open. Compare: "Did you know the time?" with "Do you know the time?"; and "Did you know that Jane's expecting a baby?" with "Do you know that Jane's expecting a baby?"
    – Mari-Lou A
    Commented Dec 1, 2013 at 8:50

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Knowing can be roughly sub-divided as knowing how to and knowing that. If the question is about knowing how to, then the normal 'rules' apply when deciding on the appropriate tense.

Do you know how to make sushi? (present tense)

When you started school did you know how to tie your shoelaces? (past tense)

When knowing means knowing that, then the past tense is used for knowledge that existed in the past:

Did you know that she was married when you asked her to dance?

You can use the present tense for present knowledge:

Do you know that the meeting has been cancelled?

However, it is also very common to use the past tense idiomatically to refer to present knowledge. So there is no problem asking:

Did you know that the meeting has been cancelled?

The past tense is often used when the questioner expects that the listener does not in fact know. For example, see these three questions from the first page of Google results for "Did you know that ...":

Did you know that he's Bob Dylan's son?

Did you know that the iPhone 2G is a collector's item?

Did You Know That Earth Is Getting Lighter Every Day?

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"Did" is a past tense verb. Therefore, saying "did you know" asks if you have previously known something. "Do" is the present tense, so saying "do you know" would ask if you currently know.

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  • I was wondering if you ever noticed that simple past is also used for additional politeness. "Did you know" in particular is extremely common when asking about the current situation.
    – RegDwigнt
    Commented Dec 1, 2013 at 11:54
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I too am annoyed by people who use

did you know

when

do you know

would more accurately reflect an inquiry.

Therefore, I urge compatriots of the English language to use do you know whenever possible.

By asking me

Did you know

you are not asking me if I know now, but you are believing that I now know but that my knowledge of a subject prior to a certain time is questionable.

By walking up to me or abruptly asking without prior discussion on the subject

did you know

tickles me as illogical use of language. Because (pls don't fault me for beginning a sentence this way), you would have failed to specify a timeline before which you believe my knowledge on the subject is questionable.

IMO, the reasonable way to use did you know is with the following similar situation,

You: Yesterday, we discussed that the white-tailed deer is actually a collective subspecies of deer.
Me: OK.
You: Did you know that the white-tailed deer is actually a collective subspecies of deer?

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    pls I find much more objectionable.
    – Mari-Lou A
    Commented Dec 1, 2013 at 8:52

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