What makes a French adjective irregular?

A feminine or plural form you can't produce by adding a letter. Petit gives petite and petits, which is regular. Doux gives douce, blanc gives blanche, and heureux gives an unchanged masculine plural, all of which are irregular.

Why is it un bel appartement and not un beau appartement?

Because beau followed by a vowel would put two vowel sounds together. French uses a special masculine form, bel, to link the words. Nouveau and vieux do the same: un nouvel hôtel, un vieil arbre.

How do adjectives ending in -eux form their feminine?

The -x becomes -se: heureux gives heureuse, sérieux gives sérieuse, dangereux gives dangereuse. The masculine plural is identical to the masculine singular, so an -eux adjective has three written forms rather than four.

Which -al adjectives don't take -aux in the plural?

Banal, fatal, final, natal, naval, and glacial take -als: des combats navals, des examens finals. Everything else follows national and gives -aux: des accords internationaux.

Do irregular adjectives also change position in the sentence?

Position is a separate question. Beau, nouveau, vieux, bon, gros, and long happen to go before the noun, while blanc, sérieux, and national go after it. Being irregular in form says nothing about where the adjective sits.

French grammar · Intermediate

Irregular Adjectives in French: The Feminine and Plural Forms You Can't Guess

Written by Camille Laurent, Lead French Curriculum Editor · Reviewed May 2026

Most French adjectives add -e for the feminine and -s for the plural. Irregular adjectives do something else: doux becomes douce, blanc becomes blanche, heureux stays heureux in the masculine plural, and beau, nouveau, and vieux have a fifth form used before a vowel.

C'est une vieille maison avec un beau jardin et de longs couloirs.

It's an old house with a beautiful garden and long corridors.

What it is

An adjective in French has to match the noun it describes in gender and number, which means every adjective has up to four forms. Irregular ones are those whose feminine or plural is not simply the masculine with a letter added, usually because the final consonant changes how it's pronounced.

beau, belle, beaux, belles ; blanc, blanche, blancs, blanches. — beautiful ; white.

How to spot it

Look at the last letters of the masculine singular. Endings in -eux, -f, -er, -el, -en, -on, -et, and -c almost always transform rather than just adding -e.

un homme heureux, une femme heureuse — a happy man, a happy woman
un vin nouveau, une robe nouvelle — a new wine, a new dress
un mur blanc, une porte blanche — a white wall, a white door
un journal national, des journaux nationaux — a national newspaper, national newspapers

The feminine form is often the more informative one: it usually shows you a consonant that the masculine has silenced.

Irregular Adjectives Quick Reference

The main irregular endings

Masculine singular	Feminine singular	Masculine plural	Feminine plural
heureux (happy)	heureuse	heureux	heureuses
neuf (brand new)	neuve	neufs	neuves
cher (dear, expensive)	chère	chers	chères
naturel (natural)	naturelle	naturels	naturelles
ancien (former, old)	ancienne	anciens	anciennes
bon (good)	bonne	bons	bonnes
blanc (white)	blanche	blancs	blanches
national (national)	nationale	nationaux	nationales

Common Irregular Adjectives Examples in French

The irregular families, with the forms you'll meet most:

-eux, -f, and -er

une histoire sérieuse: a serious story
une voiture neuve: a brand-new car
une place chère: an expensive seat
des gens curieux: curious people

The masculine plural of an -eux adjective adds nothing at all: un homme heureux, des hommes heureux.

Doubled Consonants

une bonne idée: a good idea
une grosse erreur: a big mistake
une ancienne élève: a former student
une nuit cruelle: a cruel night

Adjectives in -on, -en, -el, -eil, and -et usually double the final consonant before the -e.

Consonants That Change

une chemise blanche: a white shirt
une longue journée: a long day
une voix douce: a soft voice
une réponse fausse: a wrong answer

Blanc, long, doux, faux, roux, and sec all change their final consonant rather than adding to it: sèche, franche, douce.

Plurals in -aux

des accords internationaux: international agreements
des problèmes principaux: the main problems
de beaux paysages: beautiful landscapes
des examens finals: final exams

Most -al adjectives give -aux, but banal, fatal, final, natal, and naval keep -als.

How Irregular Adjectives Work

Almost every irregularity here is about pronunciation. French silences final consonants in the masculine and revives them in the feminine, and the spelling records that.

The Feminine -e Wakes Up the Final Consonant

In petit the t is silent; in petite it's pronounced. Irregular adjectives take this further: the consonant doesn't just wake up, it changes. Doux becomes douce, blanc becomes blanche, neuf becomes neuve.

un tissu doux, une couverture douce

a soft fabric, a soft blanket

Feminine -e = audible consonant.

Adjectives in -eux Have Only Three Forms

Heureux, sérieux, dangereux, curieux: the masculine plural is identical to the masculine singular, because you can't add -s to a word already ending in -x. The feminine replaces -x with -se.

des voyages dangereux, des routes dangereuses

dangerous journeys, dangerous roads

heureux, heureuse, heureux, heureuses.

Doubling Marks a Short Vowel

Bon becomes bonne and ancien becomes ancienne. The doubled consonant tells you the preceding vowel stays short and, in the -on and -en cases, that the nasal vowel has been undone: bon is nasal, bonne is not.

un bon repas, une bonne bouteille

a good meal, a good bottle

Nasal in the masculine, plain in the feminine.

Some Adjectives Simply Have Two Shapes

Long gives longue, sec gives sèche, public gives publique, favori gives favorite, and gentil gives gentille. These aren't derivable from a rule and have to be learned as pairs.

une école publique, un jardin public

a state school, a public garden

Learn the pair, not the rule.

-al Usually Becomes -aux in the Masculine Plural

National, principal, égal, spécial, and social give nationaux, principaux, égaux, spéciaux, sociaux. The feminine plural is untouched: nationales. A handful of adjectives resist and take -als instead.

des droits égaux, des parts égales

equal rights, equal shares

-al → -aux, feminine unaffected.

Common Mistakes with Irregular Adjectives

Incorrect: une histoire sérieux — a serious story

Correct: une histoire sérieuse — a serious story

Adjectives in -eux change to -euse in the feminine. Adding an -e to -eux is not an option, because the x has to go.

Incorrect: un nouveau appartement — a new flat

Correct: un nouvel appartement — a new flat

Before a masculine noun starting with a vowel, nouveau becomes nouvel. The form exists purely to avoid two vowels colliding, exactly as ce becomes cet.

Incorrect: une belle homme — a handsome man

Correct: un bel homme — a handsome man

Bel and belle sound almost identical but bel is masculine. Homme is masculine, so the article stays un and the adjective takes the pre-vowel form rather than the feminine.

Incorrect: des journaux nationals — national newspapers

Correct: des journaux nationaux — national newspapers

Most -al adjectives take -aux in the masculine plural. Only a short list, including banal, fatal, final, and naval, keeps -als.

The Forms That Appear Before a Vowel

Three very common adjectives have a fifth form, used only when a masculine noun beginning with a vowel or a mute h follows them directly. It's one of the details that most marks out fluent French.

Beau, nouveau, vieux, fou, mou

Before a masculine noun starting with a vowel sound, beau becomes bel, nouveau becomes nouvel, and vieux becomes vieil. The words sound like their feminine counterparts but are still masculine, and the noun's gender is unchanged.

un bel appartement: a beautiful flat
un nouvel ordinateur: a new computer
un vieil ami: an old friend
le nouvel an: the new year

The plurals are ordinary: de beaux appartements, de nouveaux ordinateurs, de vieux amis.

Only when the adjective touches the noun

The special form appears only when the adjective sits immediately before the noun. Move it after the verb and the regular masculine returns, because there's no longer a vowel to link to.

Cet appartement est beau.: This flat is beautiful.
un bel appartement: a beautiful flat
Cet homme est vieux.: This man is old.
un vieil homme: an old man

The demonstrative behaves the same way: ce livre but cet appartement, cet homme.

Des becomes de before an adjective

When a plural adjective comes before the noun, the article des shortens to de. This is why you see de beaux enfants and de longues heures rather than des beaux enfants.

de beaux enfants: beautiful children
de longues heures: long hours
de vieilles photos: old photos
des enfants intelligents: intelligent children

The rule is often relaxed in casual speech, where you'll hear des beaux enfants, but de remains the standard written form.

Irregular Adjectives FAQs

What makes a French adjective irregular?: A feminine or plural form you can't produce by adding a letter. Petit gives petite and petits, which is regular. Doux gives douce, blanc gives blanche, and heureux gives an unchanged masculine plural, all of which are irregular.
Why is it un bel appartement and not un beau appartement?: Because beau followed by a vowel would put two vowel sounds together. French uses a special masculine form, bel, to link the words. Nouveau and vieux do the same: un nouvel hôtel, un vieil arbre.
How do adjectives ending in -eux form their feminine?: The -x becomes -se: heureux gives heureuse, sérieux gives sérieuse, dangereux gives dangereuse. The masculine plural is identical to the masculine singular, so an -eux adjective has three written forms rather than four.
Which -al adjectives don't take -aux in the plural?: Banal, fatal, final, natal, naval, and glacial take -als: des combats navals, des examens finals. Everything else follows national and gives -aux: des accords internationaux.
Do irregular adjectives also change position in the sentence?: Position is a separate question. Beau, nouveau, vieux, bon, gros, and long happen to go before the noun, while blanc, sérieux, and national go after it. Being irregular in form says nothing about where the adjective sits.