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Now I know someone's already asked "Which is correct? “Did you know?” or “Do you know?”", but the question was unclear. Mine could be more specific.

Should I use "did you know" or "do you know" to introduce a fact?

I've only seen "did you know" in action. My logical deduction is that before the "question" (which is not much of a question because you're not asking for an answer), you wouldn't have been sure whether the listener'd known about what you're about to say or not. But by the time you say the "question" and introduce the fact(oid), you would know for sure they do. Why? Because you've just told them! It's like when saying "Did you know strawberries aren't berries, but bananas are?", I didn't know if you'd already known about that factoid prior to my "question", but at the time I say it onward, I know you know it because I've just told you. Therefore, it'd be reasonable to use the past tense with "did". It'd be pretty much the same with "What if I told you..."

(And I just realized I made it pretty confusing by using a lot of "know's")

However, a public speaker (who I'm supposing doesn't have a very good grasp of English because she thought a "kickass person" meant someone who encourages people to do things by kicking in their asses) told me that when you said "did you know", you implied that the listener'd already known what you were about to say, which is completely opposite to my reasoning. If I'd imply such a thing, I'd use "do you know", with a "duh" face or an annoyed tone: "Do you know/have any idea how many sleepless night I had to endure to make this and now you've just ruined it all?"

So which one should I choose to introduce a fact?

  • 2
    "Did you know that ..." is idiomatic (in US English). But "Do you realize that ..." and "Do you understand that ..." are also idiomatic (for slightly more argumentative statements). (Part of it could simply be that "Do you know" is frequently used for inquiring about ones familiarity with a person, and can be confusing in other contexts.) – Hot Licks Oct 17 '16 at 12:26
  • I agree with Shoe. – AndyT Oct 21 '16 at 11:34
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+100

A similar question was asked to the Editor of Merriam-Webster Learner's Dictionary. Serenity Carr, Assistant Editor, answered: "The phrase 'did you know' is used when you are asking someone if they know a fact, and you already know the fact." [...] The phrase 'do you know' is usually used when you are trying to find out information you do not know."

See the full explanation and some examples: http://learnersdictionary.com/qa/When-to-Use-Did-You-Know-Instead-of-Do-You-Know-

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They sentences are used differently.

If I'm talking to someone and I am going to introduce a random topic such as trivia or history, for example: I would say "did you know that XYZ"?

However if I want to find out what someone's level of knowledge on a specific subject is: I would say "do you know"?

I hope that makes sense.

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    Mind your grammar/spelling though, especially when giving answers on an English-language forum. – Vun-Hugh Vaw Oct 18 '16 at 12:40
  • What did I do wrong? – Richard Baker Oct 25 '16 at 11:13
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The two formations are used for specific types of situations.

Do you know what time it is?
Do you know how to tie complex knots?
Do you know how much that costs? (Is it expensive?)

The above are all true inquiries seeking an answer to something the asker does not know.

Did you know that tomatoes are actually a fruit?
Did you know Yoko broke up the Beatles?
Did you know how much that cost? (Because I do...)

All of these are facts that asker knows the answer to. As you rightly pointed out, the construction of a past tense is largely due to the fact that the recipient will know the information as soon as the asker completes the question. However, it is not the only used of "did you know".

Did you know her parents?
Did you know what to do when the car spun out of control?
Did you know how much that cost? (...when you bought it?)

The above are just a past tense version of the previous "do you know". Both people talking probably know the fact now, but the asker wants to determine if the fact was known at a particular moment in the past.

Do you know how long I've been waiting?
Do you know anything about computers at all?
Do you know how much that costs? (Because we totally can't afford it...)

The construction of "do you know" is also often used for the rhetorical.

Did you know that?

  • Thanks for your time put into the answer, but "Do you know what time it is?" or "Did you know her parents?" don't seem to introduce facts at all. They just sound like questions. What I was asking was about the type of "question" to which you weren't seeking an answer, i.e. fact(oid)s-introducing "questions". Therefore making these examples seem rather irrelevant and confusing. – Vun-Hugh Vaw Oct 22 '16 at 0:21
  • Vun-Hugh, the three questions that are factoids are in the second box of examples. Those are the only ones that I indicate are facts that the asker knows the answer to. They are the only construction that are acceptable for factoids. You seemed unclear where "do you know" and "did you know" should be used. I provided all of the situations for both. – Cord Oct 28 '16 at 1:15
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I always thought that "Did you know" was used to introduce trivia or other information a listener is assumed not to know.

Starting with "Do you know" might be used in a similar way, but I think starting with "Do you know" sounds, to an English speaker, like you're asking them for information you assume they have. Starting with "Do you know" may even make them feel like they are being quizzed on the information in question, even if you are not quizzing but offering information to them.

I would go with "Did you know" if I were introducing a fact to someone.

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I think that it depends on the tone of voice you use when you speak. I don't think there is a big difference semantically. Example conversation:

Person 1: "Did you know that the CEO will resign in two months?"

Person 2: "No, I did not know that!"

Other ways of introducing facts:

A little-known fact - this is useful if you are aware that the majority of the audience does not know it.

A well-known fact - is the opposite.

To my surprise or surprisingly - can also be used to introduce something new without making any assumption about the audience knowledge. The first is very humble because it says "to my surprise" while the second is general and does not directly refer to anyone in particular but draws attention to the fact.

It would be even more useful if you give a sentence that includes the fact you want to introduce.

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I think 'did you know' is used to represent past facts. Eg:'Did you know that some dinosaurs’ tails were over 45 feet long?',whereas "Do you know' is used to represent present/future facts. Eg:"Do you know that some stars produce the same enrgy in six seconds as the sun does in a year?"However, I would go with 'Did you know'.

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It depends on the context.

If you provide a statement (true or not) that the other party may or may not know, you're interested in whether or not they knew it before you told them. If they did, they still do, and if not, they now do, since you just told them. Either way, you're asking whether they did or not, hence the past tense. For example: "Did you know that spiders have eight legs?"

On the other hand, if you don't provide a fact, and instead follow up with a question to which they may or may not know the answer, what you are interested in is whether they know the answer now. In this case, use the present tense. For example: "Do you know how many legs spiders have?"

Of course, both of these can be used sarcastically or rhetorically. For example, "Did you know that would happen?" and "Do you know how stupid that sounds?"

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depending on a situation in relation to the subject tied to a certain point in time, there are at least three cases of use.

  1. Have you known that...? - No, I haven't. \ Yes I have.
  2. Did you know that...? - No, I didn't. \ Yes I did.
  3. Do you know that ...? - No, I don't. \ Yes I do.

    1. In relation to a fact of a subject which has long been passed/closed/finished.
    2. In relation to a fact of a subject that is about to close.
    3. In relation to a fact of a present active subject.
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If you use "Do you know" 1) It doesn't flow and 2) Why would the person be interested if you are introducing a fact with "Do you know", this implies that they may already know the fact before you introduce it. Let me explain my logic. If you say "Do you know?" it comes across more as a question than an introduction. If they knew the fact already, they probably won't ponder over what you presented to them. But, if you say "Did you know?", you have presented it more confidently, leading up to the fact, rather than a question asking "Do you know this fact?" Hope this helps! :D

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