Question words

Qui asks about people, que about things, and quoi is the form used after a preposition or at the end of a sentence. Quoi fais-tu is wrong: it is que fais-tu, or qu'est-ce que tu fais. In casual speech quoi does drift to the end, as in tu fais quoi, and that is fine spoken.

Casual speech keeps statement order and puts the question word where the answer goes: tu vas où ? Est-ce que goes after the question word and changes nothing else: où est-ce que tu vas ? Inversion swaps verb and pronoun: où vas-tu ? All three are correct; est-ce que is safe anywhere.

When an inverted verb ends in a vowel and the pronoun is il, elle or on, French slips in a -t-: parle-t-il, a-t-elle, va-t-on. Verbs ending in -t or -d need nothing: est-il, prend-elle. In the passé composé the pronoun swaps with the auxiliary, not the participle: a-t-il fini ?

A noun cannot swap places with the verb the way a pronoun does. Keep the noun first, then invert a pronoun that repeats it: Marie parle-t-elle français ?, tes parents habitent-ils ici ? Parle Marie français ? is wrong. After où, quand, comment or combien, when the verb has no object, a plain swap also works: où travaille ton frère ?