Ask politely and say that you don't know

A direct question to a stranger — Where is the station? — is built correctly, but it sounds sharp, almost like a demand, even with Excuse me (A1 #21). It is more polite to wrap the question in a frame: Could you tell me…? or Do you know…?. The result is an indirect question — a question put inside another sentence.

You have already seen how it is built when you reported a question (B1 #15): statement word order, no do, and if in a yes/no question. There is one difference. In a report you were telling someone about a question asked earlier, and the tense shifted. Here you are asking your own question now, so the tense stays the same as in the direct question.

The form: frame + question word (or if) + subject + verb.

Three things change at once. The word order becomes the order of a statement: first the subject, then the verb — where the station is. The auxiliaries do, does, did disappear, and their ending goes back to the main verb: does … open → opens, did … cost → cost. A question without a question word, a yes/no question, gets if or whether — this word marks the yes/no question inside the sentence.

If the question word is the subject. In the questions Who lives here? and What happened? the order is already the order of a statement, and there is no do. So nothing changes inside the frame: Do you know who lives here?, Could you tell me what happened? But in What is that building? the subject is that building, and it has to go before the verb: Do you know what that building is?

Frames — from the simplest to the softest:

I was wondering if… literally means "I was thinking about whether…", and the past tense here is not about the past: it makes the request softer, like could instead of can. I wonder… without the past is a thought said aloud — "I'd like to know…": I wonder where she is.

The question mark depends on the frame. If the frame is a question (Do you know…, Could you tell me…), there is a question mark at the end. If it is a statement (I don't know…, I wonder…, I was wondering…), there is a full stop at the end, even though there is a question word inside.

Saying that you don't know. The same indirect questions come after I don't know, I'm not sure, I can't remember, I have no idea:

if or whether. Almost always both are possible; whether is a little more formal. if here introduces a yes/no question — it means whether, not "on condition that". So it can be followed by the future: I don't know if he'll come. The rule "after if, the present instead of the future" (A2 #27) is only about conditions.

Compare with a report:

Question word + to + verb. When the question is about what to do, the clause can be made shorter: I don't know what I should do = I don't know what to do. Spanish (no sé qué hacer) and French (je ne sais pas quoi faire) have the same short form. It works with what, where, how, who, which, when and whether:

The action in this form is done by the same person as in the main part, or by "anyone". Before to only whether is possible, not if: whether to stay.

Don't say this. ✗ I don't know why to go → ✓ I don't know why I should go. With why there is no short form — only the full clause.

Check yourself. How do you ask Where's the nearest cash machine? politely? Why is there a full stop at the end of I wonder where he is? How do you say I don't know what I should say in a shorter way?