Forming a plural of an adjective in French is super easy, you add “s” at the end of the masculine or feminine form, except that the adjective ends in “-s” in its singular form which no further “s” is added in the plural.

The same is true for words ending in “-x“. Examples shown in the order of masculine singular, masculine plural, feminine singular, feminine plural.

fort, forts, forte, fortes

sympathique, sympathiques, sympathique, sympathiques

honnête, honnêtes, honnête, honnêtes

amusant, amusants, amusante, amusantes

gros, gros, grosse, grosses

nouveau (nouvel), nouveaux, nouvelle, nouvelles

vieux, vieux, vieille, vieilles