Do you know...? or Did you know..?

Of course you know that did is the past tense of do. But did you know that the question 'did you know…?' is not simply the past tense form of 'do you know?'

In English, there are two main reasons why we ask someone about what they know. 

1) One is when we don't know the answer ourselves, and we hope the other person can help us. Here are some examples of this kind of question:

• Do you know how I can get to the MTR from here?
• Do you know when this meeting will finish?
• Do you know why the train has stopped?”

When we ask questions like this, we usually don't know the answer ourselves and we need information. In other words, 'Do you know…?' acts as a request for information: "please tell me the way to the MTR from here"; "please tell me when this meeting will finish".

Notice that in all these cases, the question is formed with present tense do: "Do you know…?'

2) The other main reason we ask people about what they know is as a way of sharing information that we already know. In English we use the past form did for this:

• Your card gives you a discount on all our products and services. But did you know that it also gives you free entry to the upcoming Hong Kong Book Fair?
• Did you know that tonight's meeting has been cancelled?
• Did you know we have run out of printing ink?

in all these cases, the person asking the question already knows the information they are sharing. For example, the speaker of the last question above already knows that they have run out of printing ink. 'Did you know…?' is a way of sharing this information with someone who they think probably doesn't know yet. 

it is most common to find this expression 'Did you know…' followed by 'that'. The word 'that' introduces the information that is being shared.

As you can see from these examples, the difference between 'do you know…?' and 'did you know…?' is much more than a simple difference between present tense and past tense. The key rule to remember for most cases is:

1. Use 'do you know…?' to ask for information that you don't know
   
   
2. Use 'did you know…?' to share information that you think the person you are talking to does not know about