The fear that my sensibility is dull, inferior, is probably justified; but I am not stupid, if I am ignorant in many ways. It is important for me to do a small number of things well, rather than a wide number sketchily. That much of the perfectionist is still with me. In this daily game of choice and sacrifice, one needs a sure eye for the superfluous. It changes every day, too. Some days the moon is superfluous, some days, most emphatically not.
—  Sylvia Plath, from a journal entry
있잖아... You know!

My favourite Korean grammar pattern is ~잖다. It basically means “you know” or “remember that.” It comes at the end of a statement that you know the other person/people already knows, and you’re just reminding them of it. It’s not necessarily informal (at least not if you add the ~요 at the end) but I would advise you not to say it to your boss.

내일은 내 생일이잖아! – (You know that) tomorrow is my birthday!
언니는 아직 안 졸업했잖아요. – (You know that) she hasn’t graduated yet.
비가 많이 왔잖아요. – (You know that) it rained a lot.
술을 잘 못 마시잖아. – (You know that) I can’t drink alcohol well.

I find that ~잖다 is most often used when gently chastising your friends for forgetting something. ;)

저: 와, 오빠의 한국어는 최고! 
친구: 당연하지! 내가 한국인이잖아. ㅋㅋㅋ

*Also: 있잖아 is a common filler word when you’re thinking of what to say next. It’s like when you say “Y’know, blah blah blah” at the beginning of your sentence; it’s not necessary and it doesn’t really indicate that what you’re about to say is something they already know. It’s just a filler word.