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I know the difference in general, but it still confuses me when it comes to certain contexts, including the following:
I love a girl and I'm wondering if she knows that I love her. I'm talking about now here. Should I ask
Does she know that I love her?
Or
Did she know that I loved her?
It seems to me that the latter refers to the past event
At Quora, Richard Lueger, former editor, ESL teacher gives a reasonably comprehensive answer to the question of when to use “Do you know ...?” and when to use “Did you know ...?” [reformatted]:
Both are correct but they will usually be used in different situations....
For example, “Did you know” is commonly used [often with a that-clause] either
- to remind someone [/ make someone aware] of something, eg “Did you know that you have to pay this bill before the end of the month?”
["Did you know we're being inspected next week?"]
or
- to present some interesting fact, eg “Did you know that the Nobel Peace Prize is always awarded in Oslo, Norway?”
Of course, there are other possibilities, such as to ask for information about the past, eg “Did you know Marmaduke McGillicuddy when you were at school?”
As for “Do you know,” it’s usually used just to ask for information, eg “Do you know when the next solar eclipse will be?” / “Do you know who ate all the cookies?” / “Do you know Marmaduke McGillicuddy?” [/ “Do you know the thirteen-times table?”]
Sometimes either can be used in a rhetorical question, where the intent is to give someone a message, not to get an answer, eg “Do/did you know you’re really stupid?” and “Do/did you know that I’ve always loved you?”
I'd just add that there is also a pragmatic difference when both options are available. "Did you know that I love you?" is ... I'll say more 'guarded', tentative perhaps, than "Do you know that I love you?" And I don't think this is reasonably considered a rhetorical question!
And "Did you know that you left the keys in the ignition?" is less likely to provoke an altercation than "Do you know that you left the keys in the ignition?" (and certainly less likely than "Do you know that you left the keys in the ignition!"
If you want to ask whether somebody knows something then you could say:
Do you know that girl's name?
or
Do you know what time it is?
However if you are informing the person of something in the same breath it makes sense to say:
Did you know her name is Helen?
This is doing two things. It is informing the person that the girl's name is Helen, and asking whether they knew already. They know now, definitely, because you have just told them. But did they know already? That is the question.
In asking a girl
Did you know I love you?
you are telling her that you love her. She knows now. You are asking her whether she already knew, and it puts the onus on her to reply, perhaps on the lines of
I hoped so, because I love you.
You are correct, "Did" is past tense indeed. While "Does" derives from "Do", which is the present tense version of the same word.
present tense: I do, She does, He does, We do, They do, It does.
past tense: I did, She did, He did, We did, They did, It did.
