The shared-knowledge ending
-잖아요 is a tiny ending with a strong social signal: 'as you know' / 'you know how it is'. The speaker assumes the listener already has this information — they're invoking it, not informing. 늦었잖아요! ('It's late, you know!') is what you say when the lateness should be obvious. -잖아요 comes from -지 않아요 ('isn't it'), but in modern usage it doesn't ask — it asserts a shared truth, sometimes with a hint of 'come on, you know this'.
Forming -잖아요
Just attach -잖아요 to a verb or adjective stem (or to a copula form): • 늦다 ('be late') → 늦었잖아요 ('it's late, you know!') — past tense version is most natural here. • 바쁘다 ('be busy') → 바쁘잖아요 ('I'm busy, you know'). • 공부하다 → 공부하잖아요 ('I study, you know'). • Copula: 친구잖아요 ('they're a friend, you know'), 학생이잖아요 ('I'm a student, you know'). Past: 갔잖아요 ('we went, remember'), 봤잖아요 ('we saw it, remember').
Two main flavors
1) Reminding: 'remember? we already discussed this'. 어제 말했잖아요. ('I told you yesterday, remember?'). 민수는 못 와. 시험이잖아. ('Minsu can't come — it's the exam, you know.') 2) Justifying: 'because, obviously'. 왜 안 먹어요? — 매워서요. 한국 음식이 맵잖아요. ('Why aren't you eating? — Because it's spicy. Korean food is spicy, you know.') In both cases the speaker presupposes the hearer knows or can be reminded. It's a glue that links new claims to background fact.
Tone — careful
-잖아요 carries an edge. Used flatly with people who DON'T already know, it sounds like 'duh' or 'come on' — slightly condescending. 이거 비싸잖아요! can mean a friendly 'it's expensive, you know!' or a snippy 'this is expensive, you should have realized'. Tone and context decide. With strangers or superiors, prefer -거든요 ('it's because…' — gives reason without presuming shared knowledge) or just -아/어요.
반말 form: -잖아
Drop -요 for the casual form: 늦었잖아! ('it's late, dude!'), 시험이잖아. ('it's the exam, you know.'). Among close friends this is hyper-common — practically every other sentence in casual conversation can carry -잖아 when explaining or reminding.
Distinguishing from -잖아요? as a question
Older textbooks may show -지 않아요? ('isn't it?') as a tag question and label -잖아요 as its contraction. Modern spoken Korean has separated them: -잖아요 is a STATEMENT of shared fact, not a question. The actual tag question is still -지 않아요? or simply rising -아/어요? intonation. If you see -잖아요? with a question mark, it's usually rhetorical.
