How do I know when an adjective is masculine of feminine ?

Before considering if an adjective is masculine or feminine, it's important to remember the pronouns:il/ils (masculine) elle/elles (feminine) Equally, remind yourself that:'un' and 'le' (masculine)'une' and 'la' (feminine)
So, when it comes to understanding whether an adjective is masculine of feminine, generally you will add an -e on the end (if singular) or -es if plural.
Example: We will use the word content (happy)
Singular Masculine: ContentSingular Feminine: Contente
Plural Masculine: ContentsPlural Feminine: Contentes
ATTENTION ! (Be mindful)
When the word finishes with a consonant, the consonant is doubled before you add the -e or -es !(Masculine) Il est canadien                (Feminine) Elle est canadienne Ils sont canadiens Elles sont canadiennes
When you have understood this part of the notion, we will look into the 'exceptions of the rule'.