You use what in questions when you ask for specific information about something that you do not know.

You use what after certain words, especially verbs and adjectives, when you are referring to a situation that is unknown or has not been specified.

You use what at the beginning of a clause in structures where you are changing the order of the information to give special emphasis to something.

You use what in expressions such as what is called and what amounts to when you are giving a description of something.

You use what to indicate that you are talking about the whole of an amount that is available to you.

You say 'What?' to tell someone who has indicated that they want to speak to you that you have heard them and are inviting them to continue.

You say 'What?' when you ask someone to repeat the thing that they have just said because you did not hear or understand it properly. 'What?' is more informal and less polite than expressions such as 'Pardon?' and 'Excuse me?'.

You say 'What' to express surprise.

You use what in exclamations to emphasize an opinion or reaction.

You use what to indicate that you are making a guess about something such as an amount or value.

You say guess what or do you know what to introduce a piece of information which is surprising, which is not generally known, or which you want to emphasize.

In conversation, you say or what? after a question as a way of stating an opinion forcefully and showing that you expect other people to agree.

You say so what? or what of it? to indicate that the previous remark seems unimportant, uninteresting, or irrelevant to you.

You say 'Tell you what' to introduce a suggestion or offer.

You use what about at the beginning of a question when you make a suggestion, offer, or request.

You use what about or what of when you introduce a new topic or a point which seems relevant to a previous remark.

You say what about a particular person or thing when you ask someone to explain why they have asked you about that person or thing.

You say what have you at the end of a list in order to refer generally to other things of the same kind.

You say what if at the beginning of a question when you ask about the consequences of something happening, especially something undesirable.

If you know what's what, you know the important things that need to be known about a situation.

You say what with in order to introduce the reasons for a particular situation, especially an undesirable one.

People say 'you what?' to indicate that they do not believe or accept the remark that someone has just made, or that they have not heard or understood it properly.