One of the first phrases I learned in Korean in 괜잖아요 (Gwaenchanayo) which is almost similar to English reply ‘it’s okay’ or ‘no problem’. This is normally said when someone say thank you to you for helping that someone.
Reading this back tracked lesson from the book (which i intentionally skipped) made me realize that -잖아요 is another verb ending when attached to any plain base. It actually adds rhetorical twist to a sentece, something like ‘isn’t it obvious?’ or ‘can’t you see?’ or ‘didn’t you know?’. Here is an example converstation:
Q: 왜 이렇게 늦을까요? (Wae ireohke neujeulkkayo?) Why do you suppose he is late?
A: 비가 오잖아요! (Biga ochanayo!) It’s raining [can’t you see?]
Examining the statement (the wonder of Korean language) its actually a negative verb that transaformed into this new verb ending. Originating from -지 않아요 –> 쟎아요 –>잖아요 as the end result. Unlike the -지 ending for verb which can only be attached to plain and honorific bases, the 잖아요 ending can be attached to any bases. Here is another example:
A. 오늘 제인 씨을 만날 거에요. (Oneul Jane sshireul mannal keoeyo.) I will probably meet Jane today.
B. 그런데, 벌써 떠나셨잖아요. (Kuronde, beolsseo tteonasyeottchanayo) But she left already [didn’t you know?]
It Started with Hangul 
“ok” is 괜찮아 , not 괜잖아 ㅇ.ㅇ