Difference between What and Why

Key difference: Both ‘What’ and ‘Why’ are questions that can be used for various purposes in the English language. However, they differ in the manner that they are used and what type of answers they are seeking. ‘Why’ is used to inquire about something, or to ask for an explanation about a situation. ‘What’ is also a question, but it has many more uses that the other questions.

Both ‘What’ and ‘Why’ are questions that can be used for various purposes in the English language. However, they differ in the manner that they are used and what type of answers they are seeking.

‘Why’ is used to inquire about something, or to ask for an explanation about a situation. It is mainly used to seek the reason, cause, or purpose for something. It is more of an authoritative question. It demands an answer, as to ‘why did this happen?’ or ‘why did you or didn’t you do this.’ For example: Why is she at the party? (I want to know the reason.)

Dictionary.com defines ‘why’ as:

For what? For what reason, cause, or purpose?: Why did you behave so badly?
For what cause or reason: I don't know why he is leaving.
For which; on account of which (usually after reason to introduce a relative clause): the reason why he refused to go.
The reason for which: That is why he returned.
A question concerning the cause or reason for which something is done, achieved, etc.: a child's unending hows and whys.
The cause or reason: the whys and wherefores of a troublesome situation.
(Used as an expression of surprise, hesitation, etc., or sometimes a mere expletive): Why, it's all gone!

As ‘why’ is a standard form of a question, it is used in a standard question form, which means that the subject and verb are inverted. In a question format, the subject follows the verb, such as ‘Why did you say that?’ as opposed to a regular sentence format which is subject verb object, e.g. ‘You did say that.’

Examples:

Why did you behave so badly?
Why did you want to leave the party?
Why did you say that?
Why didn't you do your homework?
Why was she late?
Why did she go?
Why does she have to go?
Why didn't you see the movie with them?
Why did you go to Peru?

‘What’ is also a question, but it has many more uses that the other questions. ‘What’ can be used to refer to various things in various contexts. Dictionary.com lists the many contexts in which ‘what’ can be used:

(Used interrogatively as a request for specific information): What is the matter?
(Used interrogatively to inquire about the character, occupation, etc., of a person): What does he do?
(Used interrogatively to inquire as to the origin, identity, etc., of something): What are those birds?
(Used interrogatively to inquire as to the worth, usefulness, force, or importance of something): What is wealth without friends?
(Used interrogatively to request a repetition of words or information not fully understood, usually used in elliptical constructions): You need what?
(Used interrogatively to inquire the reason or purpose of something, usually used in elliptical constructions): What of it?
How much?: What does it cost?
(Used relatively to indicate that which): I will send what was promised.
Whatever; anything that: Say what you please. Come what may.
The kind of thing or person that: He said what everyone expected he would. They are just what I was expecting.
As much as; as many as: We should each give what we can.
The thing or fact that (used in parenthetic clauses): He went to the meeting and, what was worse, insisted on speaking.
(Used to indicate more to follow, additional possibilities, alternatives, etc.): You know what? Shall we go or what?
(Used as an intensifier in exclamatory phrases, often followed by an indefinite article): What luck! What an idea!
British. Don't you agree?: An unusual chap, what?
Nonstandard. That; which; who: She's the one what told me.
The true nature or identity of something, or the sum of its characteristics: a lecture on the whats and hows of crop rotation.
(Used interrogatively before nouns): What news? What clothes shall I pack?
Whatever: Take what supplies you need.
To what extent or degree? How much?: What does it matter?
(Used to introduce a prepositional phrase beginning with with ): What with storms and all, their return was delayed.
(Used in exclamatory expressions, often followed by a question): What, no salt?
What for; Why: What are you doing that for?

As can be seen by the long list, ‘what’ has many uses. It can be used to ask for a description of something or about something, (‘What is it? What does it cost?’), it may ask about a situation, (‘What is the matter? What are you doing?’), or it may be used as an exclamation or part of a sentence. (What an idea! What, no salt? We should do what we can.’) However, the confusion with ‘why’ arises when ‘what’ is used informally to ask ‘what for’.

‘What for’ is used to ask the reason for something, similar to why. For example: ‘What do we use it for? Why do we use it?’ Both will answer the same thing, however, 'what for' is an informal usage and is not grammatically considered acceptable. Still many people do use it in normal usage.

Examples: