The question mark marks the end of a question.
Question marks are used in direct questions, i.e. when the actual words of a speaker are used. A reported question should end with a full stop.
- When will we be arriving?
- Why did you do that?
- Does any of this matter?
- He’s certain to be elected, isn’t he?
Note that you put a question mark at the end of a question, even if the words in the sentence are not in the normal question order, or some words are omitted. Care is needed here as such a sentence can look, at first sight, like a statement rather than a question.
- The lady said, ‘Where are you going?’
- The lady asked where she was going.
A full stop, rather than a question mark, is used after an indirect question.
- You know he doesn’t live here any longer?
A full stop also replaces a question mark at the end of a sentence which looks like a question if, in fact, it is really a polite request.
- I’d like to know what you’ve been doing all this time.
- I wonder what’s happened.
- Will you please return the completed forms to me.
- Would you please call my brother and ask him to collect my car.