What this is
In English an adjective is a fixed word. Green is green whether it describes one car or forty women. In French it is not: the adjective takes the shape of the noun it describes, copying that noun’s gender and its number.
So vert has four written forms — vert, verte, verts, vertes — and which one you write is decided entirely by the noun, never by you. This is the single most mechanical rule in the language, and once the reflex is in place you stop thinking about it.
The rule
Start from the masculine singular, the form the dictionary gives you. Then:
| vert | ||
|---|---|---|
| masculine singular | — | un sac vert |
| feminine singular | add -e | une robe verte |
| masculine plural | add -s | des sacs verts |
| feminine plural | add -es | des robes vertes |
That covers most adjectives. The rest belong to small families with their own feminine, and they are worth knowing as a set:
| ending | feminine | example |
|---|---|---|
| -e already | no change | jeune → jeune, malade → malade |
| -eux | -euse | heureux → heureuse |
| -f | -ve | sportif → sportive |
| -er | -ère | léger → légère, cher → chère |
| -en, -on, -el | double the consonant | italien → italienne, mignon → mignonne |
| -s or -x already | no extra -s in the masculine plural | mauvais → des films mauvais |
And one rule about groups: if a group contains a single masculine noun, the adjective is masculine plural, however many feminine nouns sit beside it. Le père et la mère sont fatigués.
Why it works this way
The agreement looks like pure decoration until you say it out loud. Most French adjectives end in a consonant that is silent — and the feminine -e is precisely what makes that consonant audible again.
Petit is puh-TEE; petite is puh-TEET. Vert is VEHR; verte is VEHRT. Fort is FOR; forte is FORT. The -e is not pronounced itself; its job is to stop the preceding consonant from falling silent at the end of the word. So in speech, gender agreement on these adjectives is not a spelling detail at all — it is the difference you can actually hear.
This also tells you which agreements are inaudible. Joli and jolie, fatigué and fatiguée, and every plural -s sound exactly alike, because there was no final consonant to rescue. Roughly speaking: agreement you can hear happens after a consonant, agreement you can only see happens after a vowel.
The deeper reason French keeps all this is redundancy. As endings eroded and verb forms collapsed into each other, the language leaned harder on everything else that still carried information. Adjective agreement is one of the surviving signals of which noun you meant.
Edge cases
The final consonant is silent in the masculine and audible in the feminine
This is the one to internalise. Grand / grande, petit / petite, vert / verte, fort / forte, français / française. If you cannot hear the ending, you have said the masculine — whatever you wrote.
Adjectives already ending in -e do not change for the feminine
Jeune, malade, difficile, facile, rouge. One form covers both genders, and the plural still adds -s: des fleurs rouges. Adding a second e — une fille jeunee — is not a thing.
An adjective already ending in -s or -x adds nothing in the masculine plural
Un mauvais film → des mauvais films. Un gros chien → des gros chiens. The -s is already there and does not double. The feminine plural is unaffected by this: des chaises grosses takes its -es as normal.
A single masculine in the group makes the whole thing masculine
Marie et Paul sont contents, not contentes — one masculine subject decides it. This is a grammatical rule about words, not a claim about people, and it applies with a hundred women and one man.
Where the adjective sits is the next concept
You will notice that some adjectives here come after the noun (une robe rouge) and a few come before it (une petite maison). Both are correct, and which side an adjective takes is its own subject, coming next. For now, take the position in each example as given and put your attention on the ending.
What English speakers get wrong here
Leaving the adjective unchanged, because English never changes it. Une voiture vert is the single most common error at this stage.
Agreeing with the speaker instead of the noun. If you are a woman describing a book, the book is still masculine: le livre est petit. The adjective never reports anything about who is talking — only about the noun.
Forgetting that the plural agreement is silent. Les chiens sont noirs sounds identical to le chien est noir except for the article and the verb, so the ear gives you no warning that you have dropped the -s.
Pronouncing the masculine as if it were the feminine. Saying FORT when you have written fort is a real mistake, not a small one — it is how il est fort starts sounding like elle est forte.