French adjectives normally agree in gender and number with the noun they describe: un livre intéressant, une chose intéressante, des idées originales. This is one of the first rules learners absorb. But a specific group of pronouns breaks that pattern entirely, and understanding why requires starting from what these pronouns actually are, grammatically speaking.

Some Pronouns Have No Inherent Gender or Number

Most French pronouns carry gender and number the way nouns do. Elle is feminine singular. Ils is masculine plural. Certaines is feminine plural. When you attach an adjective to one of these, it simply agrees, the same as it would with any noun.

But French also has a small set of pronouns that refer to an unspecified person or thing without committing to any gender or number at all. They point at “someone,” “something,” “no one,” “nothing,” “anyone,” in the abstract — not at a specific, classifiable entity. This group includes:

• quelqu’un — someone
• quelque chose — something
• personne — no one (as a pronoun, not the noun “person”)
• rien — nothing
• autre chose — something else
• pas grand-chose — not much
• quiconque — anyone, whoever
• n’importe qui — anyone at all
• n’importe quoi — anything at all

Grammatically, these behave as fixed, singular, genderless units. Even though quelque chose contains the feminine noun chose, or personne looks identical to the feminine noun meaning “a person,” the pronouns themselves don’t inflect and don’t carry real gender — they’re locked expressions.

Why Direct Attachment Doesn’t Work

For an adjective to agree directly with a noun — as in une chose intéressante — the adjective needs something to agree with: a defined gender and number to match. Genderless indefinite pronouns don’t offer that. There’s no feminine or masculine form of “someone” or “something” to agree with, because the pronoun was never marked for gender in the first place.

French resolves this gap with a dedicated construction: insert de, then use the adjective in its default form — masculine singular, invariable. No agreement occurs, because there’s nothing concrete to agree with. The masculine singular functions here not as “the gender of the thing being described,” but as the neutral, unmarked default form of the adjective in French.

This produces sentences like:

• quelqu’un d’intéressant — someone interesting
• quelque chose d’important — something important
• rien d’urgent — nothing urgent
• personne de gentil — nobody nice
• autre chose de plus simple — something simpler
• pas grand-chose d’intéressant — not much that’s interesting
• quiconque de raisonnable — anyone reasonable
• n’importe qui de compétent — anyone competent
• n’importe quoi de bizarre — anything weird

Notice that the adjective never changes, regardless of who or what is actually being talked about. If you say quelqu’un de charmant about a woman, charmant still doesn’t become charmante — because it isn’t agreeing with her; it’s agreeing with the invariable pronoun construction itself.

The Contrast: Pronouns That Already Have Gender and Number

Not all indefinite-feeling pronouns behave this way. Some pronouns that translate similarly — “some,” “several,” “each,” “all” — actually do carry a fixed gender and number, and these follow the ordinary agreement rule instead:

• certains / certaines — some (certaines sont intéressantes)
• plusieurs — several (plusieurs sont arrivés en retard)
• quelques-uns / quelques-unes — a few (quelques-unes étaient vides)
• chacun / chacune — each one (chacune est différente)
• tous / toutes — all (tous sont partis)
• aucun / aucune — none (aucune n’est parfaite)

These pronouns are inflected forms in their own right — certaines is already feminine plural, chacune is already feminine singular — so an adjective describing them agrees normally, with no de inserted. There’s no gap to bridge, because the pronoun already supplies gender and number information the way a noun would.

The Underlying Principle

The real dividing line isn’t “indefinite pronoun versus regular noun.” It’s whether the pronoun carries gender and number information at all:

• No inherent gender/number → the pronoun can’t host direct agreement, so French bridges the gap with de + invariable masculine singular adjective. (quelqu’un de fascinant, rien de nouveau, n’importe quoi d’absurde)
• Fixed gender/number already built in → the adjective agrees normally, exactly as it would with any noun. (certaines sont fascinantes, chacune est nouvelle)

Once you see the distinction this way, the apparent exception stops looking arbitrary. French isn’t making an exception for “indefinite” meaning — it’s simply requiring an adjective to have something to agree with, and supplying a neutral default whenever a pronoun doesn’t offer one.