First the
was rather than
is :
Verbs of cognition (
know, realise, find out etc) are the same as verbs of reported speech (
say, tell, explain, warn, advise etc). When the cognition/speech verb is in the past, then the verb in the information clause can also be backshifted. Compare :
Exact words /Fact : I
live in Rome
He said he lived in Rome / I didn't know he lived in Rome.
Exact words / Fact : I
lived in Rome once
He said he had lived in Rome / I didn't know he had lived in Rome.
This is, however, a matter of choice. I could also say, for the first :
He said / I didn't know he lived in Rome
and for the second :
He said / I didn't know he lived in Rome.
That means though that the sentence
I didn't know he lived in Rome is, in isolation, ambiguous. Is it still true or not? However in real communication the context usually disambiguates, or the speakeer will add an adverbial or other info. to clarify :
I
didn't know he lived in Rome now.
I didn't know he lived in Rome when he was a child.
As regards
Did you know rather than
Do you know, that's more interesting.
The second form of the verb (eg
saw in
see, saw, seen) is often thought of as the "past form. But expressing past time is only one of it's uses. It can also express hypothetical events, for example : I wish I had more time...
In
The English Verb (which anyone interested in ELT should read), Michael Lewis suggests that the real concept underlying the second form is "remoteness from the here and now" . This can be remoteness in time (hence the past time use), remoteness from reality (hence the hypothetical use) or psychological remoteness.
If I say to you
Oh, Clive - I want to see you for a moment. Can you come into my office? I'm being psychologically quite direct. I can make it more tentative by saying
Oh Clive - I wanted to see you for a moment. Could you come into my office?
Another word for
remote is
distance, and we're used to thinking about the concepts of psychological distance or closeness. This just maps that concept onto grammar.
Psychological tentativeness is used to save face for the other person. A boss says to her secretary
Could you do this for me? even though she, the secretary and everyone else knows that the real meaning is
Do this! But a direct order using the imperative is not a preferred option in English (contrast this with Italian which uses the imperative far more freely). We prefer to provide the illusion that the other person has the choice, thus increasing their level of "face".
In your example, the direct
Do you know ... suggests the possibility that perhaps she doesn't know. Not knowing something is a way to lose face. Or it might be that she does know and I would be "insulting" her by suggesting she doesn't. (note that here the level of insult/loss of face is probably very, very mild, but English is a language which is far more sensitive to loss of face than most people imagine, especially British English. Americans, to the British often sound far too direct, while the Brits may seem to Americans to be faffing about without ever getting to the point). I would be more likely to say
do you know in a situation where there was no reason why the other person should know, and therefore no problem attached to not knowing :
Hi Mary. How was your holiday? Oh by the way, do you know there's a meeting this morning? Geoff called it while you were away. Even here though my "face saving instinct" pushes me to
Did you know ...
I think that's enough for one post ....
Did you know . . . ? 
Linear Mode
