when you watch something and understand what that is, we use ~는구나 or 걸. for example, when you see owesome sports star like 김연아. 멋있다 = > 멋있는 걸! or 멋있다 => 멋있구나 (는 should be out in the sentense because of grammatical reason)

걸 is so widely used in spoken Korean. #1 A : 야! 일어나서 밥 먹어! B : 아씨... 나 어제 늦게 들어왔잖아. 잘 자고 있는 걸 왜 깨우고 그래- 잘 자고 있는 걸 = who was sleeping well = B #2 A : 나 짜증나게 해놓고 지금 밥이 넘어가냐? B : 아, 배고픈 걸 어쩌라고? 배고픈 걸 a hungry state I use -ㄴ/는데, -다 for exclamations. -오, 너 오늘 좀 멋있는데? -이야, 죽인다!(slang; *ucking great/cool/nice) -ㄴ/는 걸(at the end of the sentence) sounds old-fashioned to me. About -(는)구나... in my personal lanugage, it has two usages. (I don't use it for exclamations) #1 (cellphone conversation) A : 야, 거기 ㅇㅇ(A and B's friend and hiding at A's home) 있지? B : 어어~어? 아닌데? (speaking in a hoarse tone) A : 아니긴 뭐가 아니야? 거기 있구나? S/he is there, isn't s/he? #2 A : (singing terribly) B : 아주 작곡을 하는구나. You're really re-writing the music(sarcastic; B is saying A's song is out of tune)