Here's a breakdown:
* Heureux: Means "happy" when referring to a male person or a masculine noun.
* Heureuse: Means "happy" when referring to a female person or a feminine noun.
* Heureux: Means "happy" when referring to a plural noun (regardless of gender).
Example:
* "Il est heureux." (He is happy.)
* "Elle est heureuse." (She is happy.)
* "Les enfants sont heureux." (The children are happy.)