Examine the following two questions:
Where is Dave?
Do you know where Dave is?
The first question is very direct. In an informal conversation, it’s fine, but if you are addressing the question to a stranger, you need to make it more polite, as in the second question.
There is a rule of thumb in English that the longer the statement, the more polite:
Open the door.
Please open the door.
Could you open the door?
Could you please open the door?
Would you mind opening the door for me?
Can you see how that works? The longer the request, the more polite it is. Let’s go back to our original example:
Do you know where Dave is?
Notice that this is actually two questions:
Do you know...? + Where is Dave? = Do you know where Dave is?
Note also that the subject and verb switch places in the second part of the question.
If it is a yes/no question, we use ‘if’ or ‘whether’:
Do you know? + Is Dave here = Do you know if Dave is here?
Do you know? + Is Dave here = Do you know whether Dave is here?
As well as ‘Do you know’, we can use:
May I ask whether this seat is taken?
Could you tell me where the sink is?
Would you know where I could buy some printer ink?
I’d like to ask whether you have met your KPIs.