生日快乐(shēng rì kuài lè) - happy birthday in Mandarin

生日快乐 (shēng rì kuài lè) is the standard birthday greeting, and the word order is the first thing to fix. Chinese names the occasion and then the wish, so it reads as "birthday happy". The English order 快乐生日 is never said.

The same frame builds every other festive greeting you will need: 新年快乐 (happy new year), 圣诞快乐 (happy Christmas), 节日快乐 (season's greetings). Learn the shape once and it does the whole calendar.

To make it warmer, put 祝你 (zhù nǐ, I wish you) in front: 祝你生日快乐 is what people say out loud and what the song sings. The bare four characters are what goes on the cake.