These sentence-final endings add attitude rather than information: -거든요 supplies a reason the listener doesn't know yet, -잖아요 reminds them of something you both already know, and -네요 reacts with fresh surprise or admiration at what you've just noticed. Choosing the wrong one is grammatical but socially off.
Examples
- 어제 못 갔어요. 일이 많았거든요.
- I couldn't go yesterday - I had a lot of work, you see.
- 그건 아까 말했잖아요.
- But I told you that earlier, remember?
- 한국어를 정말 잘하시네요!
- Wow, you speak Korean really well!
The mistake to avoid
Using -잖아요 with someone senior or with information they couldn't know - it sounds like scolding.
Where this fits in
표현과 뉘앙스, Korean C1: Master register, idiomatic expression, and conveying fine emotional nuance.