Did you know ... or do you know ....?

Question

I always find it hard to understand why native speakers of English say "Did you know that Thomas Edison was the inventor of the light bulb?" .... why do they not say "Do you know that Thomas Edison was the inventor of the light bulb?" ... When a person asks this kind of question, I think he or she asks about someone's present knowledge, not his or her past knowledge.

Thanks

KrisBlueNZ · Accepted Answer

"DID you know" is appropriate because you're about to tell the person something, and you want to know whether the person ALREADY knew it BEFORE you told them.
I can rephrase the question to make it even more obvious.
"Edison invented the electric light bulb. DO you know that?"
"Edison invented the electric light bulb. DID you ALREADY know that?" (or "DID you PREVIOUSLY know that?")
The first sentence doesn't make much sense, because in the process of asking the question, you are giving the information, so once you've finished asking the question, the only sensible answer is "yes", because the person now knows the fact, because you've just told him.

Widi Nugroho · Answer

Thank you very much Kris for replying to my question. Now I got it. So, when do we use "Do you know ....? .. can you give me one or two examples? Thanks ...

Regards · Answer

Hi,
It is one of those idiosyncrasies of English non-native speakers often have trouble understanding.
In fact, both questions carry the same meaning and mainly refer to the present.
The first question is formally correct. The verb do is changed to did because the verb was occurs after it.
In terms of grammar, we name this process backshift. The meaning can refer to the present although the verbs
in question are used in their past forms. Producing sentences and questions like this one is quite natural
and occurs often in speech.

Here are other examples:
I didn't know she is not his mother ~ I didn't know she was not his mother.
I thought Doris is a generous person ~ I thought Doris was a generous person.
Mike said he is in France ~ Mike said he was in France.

Regards

Vsuresh · Answer

Hi Regards
Your explanation is informative. Thank you.
I have a doubt about this back shift. That is we do in reported speech even though sometimes it is not realistic.
In the examples you have given, for instance third sentence, " Mike said he was in France."
This doesn't sound that odd, but the first one " I didn't know she was not his mother." sounds odd.
I know that we hold to back shift. Don't you think it stifles spontaneity sometimes?

Did you know ... or do you know ....?

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