Based on the level of respect or formality, you can say "Happy birthday!" in Korean in 3 ways:

• Informal: saengil-eul chugha-hae! (생일 축하해!)
• Formal: saengil-eul chugha-hamnida! (생일 축하합니다!)
• Very formal: saengsin-eul chugha-deurimnida! (생일 축하드립니다.)