可以说"생일 축하합니다(saeng-il chuk-ha-ham-ni-da)" \^o^/

생일축하해! Saeng il chuk ha hae! 생일축하합니다. >> it's a polite expression.. Saeng il chuk ha hap ni da (Saeng il chu ka ham ni da, actually!)

Saengil Chukhahae (Talking to a friend; Informal) Saengil Chukhahaeyo (Polite) Saengil Chukhahamnida (Very formal)