~는 중이다 and ~고 있다 They are synonyms, but there are cases where one is preferred over the other. As I see it, ~는 중이다 is more explicitly "in the process/middle of -ing", which always suggests something ongoing. -고 있다 on the other hand can also mean "in certain state/action" which may not mean actively going on. Here are examples where the two make little difference. - 나 지금 가는 중이야 = 나 지금 가고 있어. I am on my way there right now. - 저는 (밥을 / 저녁을) 먹는 중이에요 = 저는 먹고 있어요. I am eating. - 지금 식사 중이야 = 지금 식사하고 있어. (same) These examples are about ongoing events, so ~ 중이다 is preferred. - 1951년 한국은 전쟁 중이었다: Korea was in war in 1951. (전쟁하고 있었다 sounds unnatural) - 수업 중(in class), 가동 중(in operation), 방송 중(on air): common set noun phrases implying a process. The following are not so much about an explicit process, so -고 있다 sounds more natural. - 잘 지내고 있니? = 잘 지내니? Is everything going well with you? - 이 꼬마들(이) 지금 싸우고 있다 = 이 꼬마들 지금 싸운다: These kids are quarreling now.