les adjectifs irréguliers

Regular adjectives are formed by adding an e to the masculine form in the singular (content / contente), or by adding an s to the masculine and feminine forms in the plural (Francisco et Edouard sont contents / Anna et Julia sont contentes). 

This group of adjectives is by far the most common. BUT there are also irregular adjectives.

Irregular adjectives do not have the normal -e, -s, or -es endings. 

Some adjectives are completely irregular: you just need to memorize them.  There are not very many!

Some adjectives have identical masculine and feminine forms. This is generally the case with adjectives ending in e to their masculine form and with foreign adjectives like 'snob', 'cool', etc. 

For example: Aman est un homme calme; Maria aussi est calme. Ils sont calmes. (Aman is a calm man; Maria is also calm. They are calm.) 

number  (singular/plural)

The majority of adjectives are regular in the plural; an -s is added to the singular masculine or feminine forms. 

There are two major exceptions to this rule:

1. Adjectives that end in S or X. The masculine singular and plural  forms are identical: un animal heureux (a happy animal), des animaux heureux (happy animals). 

2. However, the feminine plural form of these adjectives is regular; it is formed by simply adding an s to the feminine singular form: une fille heureuse (a happy girl), des filles heureuses (happy girls).

3. Adjectives ending in al in the masculine singular form change to aux in the masculine plural form.