니까 모르다

you don't realise it, because ~

-(으)니까 supplies the reason for not knowing or not noticing. The characteristic use is telling someone why they are blind to something obvious: 너는 네 얼굴을 매일 보니까 모르지! (you see your own face every day, that's why you don't realise), 체력이 되니까 몰라 (you've got the stamina, so you don't notice). It also covers plain inexperience — 처음 타니까 모르잖아 (it was my first time flying, so of course I didn't know).

Register: casual