How to say and write happy birthday in korean?

Hm~ Our Korean Language have 1 simple rule, which is the age rule. Haha. You can see the difference after reading this~ Study time~
In polite language (to older person)

생일 축하드려요.
Saeng-il chu-ka-d-ryeo-yo.
In friendly language (to a friend or younger person)
생일 축하해.
saeng-il chu-ka-hae.
We need to show our politeness and respects using the ending 요, 하세요, 드려요 to somebody who is older than us. The main topic is 생일 축하 (Happy Birthday).
But we dont say that to our parents or grandparents and boss like, "Mom, 생일 축하!"
there is no sense of expression in there at all!
So when you talk to elders, make sure you use the polite way.
But for friends,
we can talk freely most of the time.
Like,
"키, 생일 축하해~ (Key, Happy Birthday~)"
Because friends are no stranger to us,
and we can talk anything directly~

Learn how to read in Korean here.