생일축하드립니다 - saeng il chuka deu rib ni da (formal) 생일축하합니다 -saeng il chuka hab ni da (formal) 생일축하해요 - saeng il chuka hae yo (polite, casual) 생일축하해 - saeng il chuka hae (casual) 생일축하드려요 - saeng il chuka deu ryeo yo (polite, casual)

생일축하합니다 (saeng il chuk ha ham ni da ) 생일 = Birthday 축하 하다 = To congratulate

생일 축하합니다 - formal, to be used with people much older than you 생일 축하해요 - still formal, but less than the above. I think this is the most common. 생일 축하해 - informal, used with people you are very close friends with and/or younger than you