게임 (놀다 - 하다) First, 놀다 is an intransitive verb meaning "play (in general)" or "have fun". We never say 게임을 놀다. To play a game is always expressed as 게임을 하다. Sometimes we say 게임을 즐기다 (enjoy a game), but never 놀다. (~을 놀다 exists as an archaic form, but it's not used in modern Korean anymore) With that in mind, we can check your phrases: - 형이 심시티를 놀고 있는 중이었다 => 형이 심시티을 하고 있는 중이었다 = Hyung was in the middle of playing SimCity. (놀고 was the only thing unnatural). - 형이 심시티를 하면서 놀고 있던 중이었다 = Hyung was (in the middle of) having fun playing SimCity. (~하면서[하며] 놀다, "have fun playing/doing ..." is a common expression. ~를 가지고 놀다 = "play with (a toy)" is another) Both 놀고 있는 중이었다 and 놀고 있던 중이었다 are okay. They mean the same thing with a slight nuance difference. The first emphasizes the action of playing while the second focuses more on the fact that it is now past. You could also simplify it to 형이 심시티을 하면서 놀고 있었다. ~던 indicates either a single activity/state that lasted a while(내가 먹던 사과, 내가 하던 일) or something that occurred repeatedly. So it's not limited to the meaning of "used to" in the sense of habitual past activities. So 놀고 있던 중이었다 is not wrong (but it sounds a bit verbose). ~면서 and ~ㄴ/는/ 중이다 has no overlap in meaning, so they are used together without sounding redundant. ~면서 only indicates two things are happening in parallel. ~중이다 just means something is ongoing.