잖아(요) is used to show that information should already be known to the other person. It roughly equates to ‘as you know’, ‘you should know that…’ or ‘I told you that…’:
Come on… you know.
You know that it is!
You know I’m right!
– 발렌타인 데이를 즐거워했으면 좋겠어요.
– I hope you enjoy Valentine’s Day.
– 남자친구 없잖아!
– You know I don’t have a boyfriend!
– 오늘밤 삼겹살을 먹을까요?
– Honey, shall we eat barbeque pork tonight?
– 아니… 고기를 안 먹잖아!
– No… you know I don’t eat meat!
– 잠깐 – 오늘은 어머니의 생일이에요?
– Wait – it’s your mother’s birthday today?
– 이전에 말했잖아!
– Yes I told you earlier!
– 오늘은 왜 교회에 가요?
– Why are you going to church today?
– 오늘 일요일이잖아요.
– Come on… it’s Sunday today!
– 왜 아주 많은 선물을 사요?
– Why are you buying so many presents?
– 다음 주에 크리스마스이잖아!
– You know it’s Christmas next week!