Every French noun carries a grammatical gender and a number, and adjectives have to agree with both â masculine nouns pair with an adjective's base form, feminine nouns usually add an -e, and either way, a plural noun adds an -s on top of whichever gender form you started with. Most adjectives follow that predictable pattern, but a handful of the most common ones â beau, vieux, nouveau â break it twice over: they change enough between masculine and feminine to be worth learning as sets, including a rarely-taught third form used only right before a masculine noun that starts with a vowel sound, and in the plural, beau and nouveau swap the expected -s for an -x instead.
Masculine nouns take the adjective's base form. Feminine nouns usually add -e, though the exact change depends on how the masculine form ends. Either way, a plural noun then adds an -s on top of whichever gender form you started with -- or, for a few adjectives like beau and nouveau, an -x instead.
Base form of the adjective, +s for the plural
Usually the masculine form + -e, +s for the plural
C'est un vieil ami, pas une vieille amie.
He's an old (male) friend, not an old (female) friend.
Beau, nouveau, and vieux each have a special third form -- bel, nouvel, vieil -- used only right before a masculine noun that starts with a vowel sound, purely so the two words flow together. It's still the masculine form (ami stays a male friend), just spelled to avoid a vowel collision; vieille is the separate feminine form, used for amie regardless of what sound follows. The same three adjectives carry a second irregularity into the plural: beau and nouveau take -x instead of -s (de beaux jardins, de nouveaux amis), while vieux, already ending in -x in the singular, just stays vieux (de vieux amis).
Take the quiz and see if you can match adjectives to the noun they're describing.