How do you make a French adjective feminine?

The default is add -e to the masculine form: petit -> petite, grand -> grande, francais -> francaise. Adjectives already ending in -e don't change (rouge, jeune, facile stay the same in both genders). Then there are six predictable patterns: -x -> -se (heureux -> heureuse), -el -> -elle (naturel -> naturelle), -en -> -nne (italien -> italienne), -f -> -ve (actif -> active), -er -> -ere (premier -> premiere), and -on -> -nne (bon -> bonne). Beyond that, a small set are irregular and have to be learned per word (beau / belle, vieux / vieille, nouveau / nouvelle, blanc / blanche).

Where does a French adjective go - before or after the noun?

After the noun is the default. 'Une voiture rouge', 'un livre interessant', 'une fille intelligente'. About a dozen common adjectives go before the noun instead - the BAGS set: Beauty (beau, joli), Age (jeune, vieux, nouveau), Goodness (bon, mauvais), and Size (grand, petit, gros, long, haut). 'Une belle voiture', 'un nouveau livre', 'un petit chat'. A handful change meaning depending on position - ancien before a noun means former, after it means ancient; grand before means great, after means tall. Foundation tier sticks with the position rules; meaning shifts are Higher-tier.

French Adjective Agreement

French adjectives have four forms - masculine singular, feminine singular, masculine plural, feminine plural - and they agree with the noun on both axes.

The four-form pattern

Masc sg	Fem sg	Masc pl	Fem pl
petit	petite	petits	petites
grand	grande	grands	grandes
francais	francaise	francais	francaises

Build the feminine from the masculine, then add -s for the plural of either. Adjectives ending in -s or -x already don't change in the masculine plural (francais -> francais).

The feminine endings

The default is add -e. Then there are six predictable shortcuts:

Masculine ending	Feminine ending	Example
default	+ e	petit -> petite
-e (already)	no change	jeune -> jeune
-x	-se	heureux -> heureuse
-el	-elle	naturel -> naturelle
-en	-nne	italien -> italienne
-on	-nne	bon -> bonne
-f	-ve	actif -> active
-er	-ere	premier -> premiere

These shortcuts cover most adjectives you'll meet. A small irregular set has to be learned per word: beau / belle, vieux / vieille, nouveau / nouvelle, blanc / blanche, sec / seche, doux / douce, frais / fraiche, long / longue, gros / grosse, faux / fausse.

The plural

Default: add -s (petite -> petites, grand -> grands).
Already ends in -s or -x: no change in the masculine plural (francais -> francais, heureux -> heureux). Feminine plural still adds -s (francaises, heureuses).
Ends in -al: masculine plural becomes -aux (national -> nationaux, principal -> principaux). Feminine plural is regular (nationales).
Ends in -eau: masculine plural adds -x (beau -> beaux, nouveau -> nouveaux). Feminine plural still adds -s (belles, nouvelles).

Position: after the noun by default

Most French adjectives go after the noun.

une voiture rouge (a red car)
un livre interessant (an interesting book)
une fille intelligente (an intelligent girl)
un homme francais (a French man)

The BAGS set: before the noun

A small closed group sits before the noun. The mnemonic BAGS covers them:

Beauty: beau, joli
Age: jeune, vieux, nouveau
Goodness: bon, mauvais
Size: grand, petit, gros, long, haut

Examples:

une belle voiture (a beautiful car)
un nouveau livre (a new book)
un bon ami (a good friend)
un petit probleme (a small problem)

When two BAGS adjectives stack, the order is flexible: une belle petite maison or une petite belle maison are both heard, with a slight preference for size + beauty.

The vowel-buffered masculine forms

Three BAGS adjectives have a third masculine singular form used before a vowel or silent h:

Masc sg	Before vowel	Fem sg	Masc pl	Fem pl
beau	bel	belle	beaux	belles
vieux	vieil	vieille	vieux	vieilles
nouveau	nouvel	nouvelle	nouveaux	nouvelles

un beau garcon but un bel homme
un vieux chien but un vieil ami
un nouveau livre but un nouvel ordinateur

Same pattern as cet vs ce - bridge the vowels.

Worked examples

Le petit chat noir dort. (The small black cat is sleeping.)
Une belle journee ensoleillee. (A lovely sunny day.)
Mes amis francais habitent a Lyon. (My French friends live in Lyon.)
C'est une vieille histoire. (It's an old story.)
Ce sont de bons restaurants. (Those are good restaurants.)
Elle porte une robe blanche et des chaussures noires. (She's wearing a white dress and black shoes.)

Common mistakes English speakers make

Forgetting to agree the adjective in writing: les filles intelligent instead of les filles intelligentes. The plural -s is silent in speech but mandatory in writing. Putting every adjective after the noun and writing un livre nouveau when it should be un nouveau livre. And forgetting the irregular masculine-before-vowel forms - un beau homme is wrong, it's un bel homme.