How to Translate "What?" Into French

French learners often have trouble deciding how to translate "what" into French. Should it be que or quoi, or maybe that pesky quel? Understanding the difference between these terms is critical to know how to use them correctly.
The problem with translating "what" into French is that it has numerous grammatical functions in English. It can be an interrogative pronoun or adjective, relative pronoun, exclamative adjective, adverb, or object of a preposition, and may be found in any position in a sentence. In contrast, French has different terms for most of these possibilities, including que, qu'est-ce qui, quoi, comment, and quel. In order to know which term to use, you need to understand what function each of them performs.

Asking a Question

When asking a question with "what" as either the subject or object, the French equivalent is the interrogative pronoun que.

As the object of a question, que may be followed by either inversion or est-ce que:

Que veux-tu? Qu'est-ce que tu veux?
What do you want?

Que regardent-ils ? Qu'est-ce qu'ils regardent ?
What are they watching?

Qu'est-ce que c'est (que ça)?
What is it/that?

When que is the subject, it must be followed by est-ce qui. (Don't let that qui fool you into thinking this means "who"; in this type of construction, qui is simply acting as a relative pronoun with no actual meaning of its own.)

Qu'est-ce qui se passe?
What's happening?

Qu'est-ce qui a fait ce bruit?
What made that noise?

To ask a question in which "what" comes after the verb, use quoi. Note that this is an informal construction:

Tu veux quoi?
You want what?

C'est quoi, ça? Ça c'est quoi?
What's that? (Literally, That's what?)

When "what" joins two clauses, it is an indefinite relative pronoun.

If "what" is the subject of the relative clause, use ce qui (again, this doesn't mean "who"):

Je me demande ce qui va se passer.
I wonder what's going to happen.

Tout ce qui brille n'est pas or.
All that glitters is not gold.

When "what" is the object, use ce que:

Dis-moi ce que tu veux.
Tell me what you want.

Je ne sais pas ce qu'elle a dit.
I don't know what she said.

When "what" precedes or otherwise modifies a noun, you need to use quel (which literally means "which"), and can be either an interrogative adjective or an exclamative adjective:

Quel livre veux-tu? Quel livre est-ce que tu veux?
What (which) book do you want?

À quelle heure vas-tu partir?
(At) What time are you going to leave?

Quelles sont les meilleures idées?
What (which) are the best ideas?

Quel livre intéressant!
What an interesting book!

Quelle bonne idée!
What a good idea!

Prepositions: Then What?

When "what" follows a preposition, you usually need quoi in French.

In a simple question, use quoi followed by either inversion or est-ce que:

De quoi parlez-vous ? De quoi est-ce que vous parlez ?
What are you talking about?

Sur quoi tire-t-il ? Sur quoi est-ce qu'il tire ?
What is he shooting at?

In a question or statement with a relative clause, use quoi + subject + verb:

Sais-tu à quoi il pense?
Do you know what he's thinking about?

Je me demande avec quoi c'est écrit.
I wonder what it's written with.

When a verb or expression requires de, use ce dont:

C'est ce dont j'ai besoin. (J'ai besoin de...)
That's what I need.

Je ne sais pas ce dont elle parle. (Elle parle de...)
I don't know what she's talking about.

When à is the preposition and it is placed either at the beginning of a clause or after c'est, use ce à quoi:

Ce à quoi je m'attends, c'est une invitation.
What I'm waiting for is an invitation.

C'est ce à quoi Chantal rêve.
That's what Chantal dreams about.

And finally, when you didn't hear or didn't understand what someone just said and you'd like them to repeat it, use the interrogative adverb comment, which is considered nicer than saying "quoi".