What is 'Be happy' in French?

Sois heureux is a French equivalent of 'Be happy'. The imperative 'sois' means '[you] be'. The masculine adjective 'heureux' means 'happy'. Together, they're pronounced 'swah-zuh-ruh'. 'soyez heureux' is the same thing but for the imperative, 2nd person plural.

That's what's said to a male. For a female, it's 'heureuse'. So the phrase is pronounced 'swah-zuh-ruhz'.