-잖아요
ja-na-yo
Pointing again at what is already on the table: -잖아요
The picture A finger pointing again at the thing already lying on the table between you. -잖아요 is a finger on what you both already know; -거든요 is a hand bringing out what the other person has no way of knowing.
Where the picture shows up in -잖아요
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pointing back at what you said
어제 말씀드렸잖아요.
eo-je mal-sseum-deu-ryeot-ja-na-yo
I told you about it yesterday.
Pushing yesterday's document back into the middle of the desk and tapping it with a fingertip
Depending on tone this can sound like a complaint. With a senior, 말씀드렸는데요 is safer.
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a date you both can see
오늘 토요일이잖아요. 문 닫아요.
o-neul to-yo-i-ri-ja-na-yo. mun da-da-yo
It's Saturday today, so they're closed.
Pointing at the Saturday square on the wall calendar so you both look at it
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the order of things anyone knows
약을 드셔야 낫잖아요.
ya-geul deu-syeo-ya nat-ja-na-yo
You only get better if you take the medicine.
Holding the pill up on your palm in front of them, tapping the obvious next step
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my situation, which they know
제가 운전을 못 하잖아요.
je-ga un-jeo-neul mot ha-ja-na-yo
You know I can't drive.
Setting a fact about yourself you have mentioned many times back in front of them
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persuading with shared knowledge
거기 비싸잖아요. 다른 데 가요.
geo-gi bi-ssa-ja-na-yo. da-reun de ga-yo
That place is expensive. Let's go somewhere else.
Bringing the price list you both saw back to mind, then turning your steps another way
All of it in one picture
-잖아요 is a single pointing finger. Point at yesterday's words and you get 말씀드렸잖아요; point at the calendar and you get 토요일이잖아요; point at a sequence everybody knows and you get 낫잖아요. Point at a fact about yourself they already have and it becomes the reason behind a favour; point at the price list you both saw and it becomes persuasion. It never brings out something new — it makes you look again at what is there.