여기는 어쩐 일이야? = 여기에 (네가) 온 것은 어찌된 일이야? = What brought you here? 어쩐(a) means 어찌된 (from 어찌되다, 어찌 = 어떻게), meaning 'something happens in a certain way'. It is used in question or negative phrases. - 네가 어찌되든 상관없다 = It doesn't matter what happens to you. - 밤 1사가 넘어서 들어오다니 어찌된 일이냐? = You're home past one in the morning - what happened? 둘이 아는 사이야? = Do you already know each other? 사이(n) means 1) a gap/distance between two things, 2) relation/terms between two people. - 그 친구와 저는 좋은 사이에요 = He and I are on good terms. - 둘은 이 일로 사이가 나빠졌다 = The two had a falling out because of this. 너보다 훨씬 잘 아는 사이야, 내 남친이야. = He and I are much closer than you and I. He's my boyfriend.