quel, quelle, quels, quelles

What this is

Every question word in this module so far — qui, que, où, quand, comment, pourquoi, combien — is invariable, the same spelling no matter what it’s asking about. quel breaks that pattern: it’s an adjective, and like any adjective in this course, it agrees in gender and number with the noun it questions.

The rule

Four spellings, one word:

Quel restaurant cherches-tu ?     Which restaurant are you looking for?   (m. sg.)
Quelle ville habites-tu ?         Which city do you live in?             (f. sg.)
Quels problèmes avez-vous ?       What problems do you have?             (m. pl.)
Quelles villes visitez-vous ?     Which cities are you visiting?         (f. pl.)

This is exactly the adjective agreement from M4 — the agreement rule doesn’t change just because the adjective happens to be a question word.

quel + noun asks directly. quel est / quelle est + noun phrase asks through être, when you want to identify something rather than just name it:

Quel restaurant cherches-tu ?   (direct — restaurant is the object of cherches)
Quel est le problème ?          (through être — problème is the noun being identified)

quel can also combine with the frames you already know, including est-ce que for object-position questions:

Quel restaurant est-ce que tu préfères ?   Which restaurant do you prefer?

Why it works this way

quel questioning a noun is grammatically no different from any other adjective describing one — grand jour, quel jour; petite ville, quelle ville. French doesn’t carve out a special invariant form for question-adjectives the way it does for question-adverbs (où, quand, comment never change); quel simply follows the same agreement machinery as every other adjective in the language, because structurally, that’s exactly what it is.

Edge cases

Four spellings, two sounds

quel and quelle are pronounced identically; so are quels and quelles. In speech, gender and number are carried entirely by what comes after quel — the noun’s own sound, the article, sometimes nothing detectable at all until context resolves it. This is the same phenomenon as les hiding gender in the plural, back in M1.

quel est agrees with the noun, not the question

quel est le problème and quelle est l’idée both agree with the noun that follows être (problème is masculine, idée is feminine) — there’s no hidden subject or abstract “question” for quel to agree with instead. Whoever is asking, whatever they’re asking about, the agreement target is always the noun in the sentence.

What English speakers get wrong here

English “what” and “which” are invariant, so the instinct is to treat quel the same way and leave it unchanged regardless of the noun that follows — producing quel ville or quel idées instead of quelle ville and quelles idées. Since quel/quelle and quels/quelles sound the same, this mistake is invisible in speech and only shows up in writing, which is exactly why it’s worth checking deliberately rather than trusting your ear.