N-(이)잖아(요) / V/A-잖아(요)
present something as obvious or evident
Form
Noun + -(이)잖아(요); verb/adjective + -잖아(요).
Function
Presents the content as something the speaker treats as evident, already known, or now plainly confirmed. In 어? 정말 사람이잖아!!, Sara points in surprise to what the visual evidence confirms: “Oh—there really is a person!”
Why this form?
-(이)네(요) — “oh, it is… / I see that…”: simply marks a discovery as it is made; -(이)잖아(요) additionally carries “look, it really is / it is obvious now.”
-(이)군(요) — “ah, I see that it is…”: expresses a more reflective realization or observation; -(이)잖아(요) is more engaged and can sound like a reminder or an obvious fact being pointed out.
Register and nuance
The exact force depends heavily on intonation. The form can remind someone of shared information, object or correct, or highlight a realization that has become obvious.
Forms not to confuse
-(이)네(요)
정말 사람이네요! — Oh, it really is a person!
-(이)군(요)
여기가 출구군요. — Ah, I see the exit is here.
Examples
어? 정말 사람이잖아!! — Oh! There really is a person!!
오늘은 휴일이잖아요. — But today is a holiday, you know.