What

interrogative pronoun (3rd person singular inanimate), fused relative pronoun, interrogative determiner / adjective.

Pronoun

In both English and French, it is considered quite lax to say:

• *You want what?
• Tu veux quoi?

But I am sure that both anglophones and francophones find these phrasings perfectly functional. Nevertheless, normatively it does seem to me that the English proposition is considered "wrong" despite its relative frequence in natural speech. Of course in school we are taught to say:

• What does she want? // What do they want?
   wədə(z/ʊ)
• Qu'est-ce qu'elle veut? // Que veulent-elles?
    k(esk)(ə)

There are two parts to what. The first part (wh-) is found in who, how, why, when, etc. and represents an "information gap" (a lack of info), the second (-at) is found in that and represents an object at a certain distance from the speaker. This object can also be an object of discourse, and in some cases functions as a grammatical subject:

1. What matters is that we keep trying.
   Ce qui importe c'est que nous continuons d'essayer.
2. What's strange is that I didn't get what I wanted.
   Ce qui est bizarre, c'est que je n'ai pas eu ce que je voulais.

(In many of the above examples, "what" is referred to as a fused relative pronoun (it is a pronoun which "functions as its own antecedent"), and in the case of #1 its role is simply to hold the place for a long subject to be stated later (that we keep trying). This structure is known as a WH-cleft or pseudocleft sentence (very useful despite its ugly name, as we shall see in the next section). In the first series of examples "what" was, on the other hand, an interrogative pronoun. In the following examples, it is called an interrogative indirect pronoun.) These names are included only for completeness.

• They asked what we were doing _what .
• She imagined what the other girls were doing _what.

WH-cleft sentences

An extremely common rhetorical device to draw attention to the semantic object is the so called WH-cleft or pseudo cleft structure (in #2 below)

1. I need a dollar.
2. (What) I need (is) a dollar.
3. I need a dollar, a dollar is what I need.
4. I need a dollar. A dollar, that's what I need. (see Aloe Blacc ♬)

The basic formula of such sentences is (what) X (be) Y. An inverted WH-cleft follows a similar pattern: Y (be) (what) X. Some examples:

• What I want is a day off! ⇨ A day off is what I want.
• What we finally agreed on was a more flexible delivery date. ⇨ A more flexible delivery date was what we finally agreed on.
• What the customer said was that she was dissatisfied.      inversion is always awkward with "that" because English prefers to put "heavy" clauses at the end of the sentence.

If you are an attentive reader, you have just discovered a method of tripling the number of grammatical sentences you know how to say:

e.g.

1. I want a pizza. ⇨ What I want is a pizza. ⇨ A pizza is what I want.
2. I need more time. ⇨ What I need is more time. ⇨ More time is what I need.
3. I can't believe they cancelled their order. ⇨ What I can't believe is that they cancelled their order. ⇨ That they cancelled their order is what I can't believe.

Though it is possible to omit "that" from #3 in informal, relaxed speech, it is considered necessary in writing when introducing a stand-alone sentence. (They cancelled their order.)

Interrogative Determiner / Adjective

In modern English grammar this is generally called an interrogative determiner. In traditional French & Latin grammar it is referred to as an interrogative adjective.

• What time is it?
• What kind of camera did you buy?
• What color is your car?

This last sentence can be compared to:

• Which color do you like best?

where there is a limited list of colors possible. For example, you may be asking for a friend's to help you chose between three pairs of boots: black, brown and bordeaux...

On the other hand, if you ask a more general question, "what" is used instead of "which".

• What is your favorite color?

Predeterminer / Intensifier

• What a mess!
  Quel bazar! // Quel bordel! (2e = à ne pas utiliser, vulgaire, contrairement à l'anglais)
• What a hassle!
  Quel bazar! // Quel emmerde! (2e = à ne pas utiliser, vulgaire, contrairement à l'anglais)