Korean grammar, 잖아 vs 거든, 잖아요 vs 거든요



From Korean TV program "Radio Star"

 잖아 and 거든(2) have common usage indicating "reason", that's why students are confused which one to use when they have to say the reason of their action or state.

잖아요 :  reason, confirmation, proposal
거든요 :  reason, topic

The biggest difference is, "잖아요" gives a reason the listener already knows, while "거든요" 's listener doesn't know the information before.

See the examples below :
한국어를 잘하시네요. (Your Korean is good.)
-제가 한국어 공부를 열심히 하잖아요. (I study Korean very hard. (as you already know))
-제가 한국어 공부를 열심히 하거든요. (I study Korean very hard. (new information))

***But please pay attention when you use one of the expressions with intonation.

저랑 어제 약속했잖아요~! (You made a promise to me yesterday! (Why did you forget?))
나는 숙제 벌써 다 했거든? (I already did my homework.(Are you getting mad to hear that? :p))

"잖아요" could have nuance when you say it with a strong accent that the speaker is telling off the listener like 'Why are you saying that?' or 'Didn't you know that?'.
"거든요" could have nuance when you say it with rising intonation that you are showing off the fact.