-ㄴ 걸 It acts as ending a sentence by attaching to a verb or a adjective in colloquial expression. 나는 오빠를 좋아한다.( a sentence preferred in writing) 나는 오빠가 좋은걸( one of the sentences preferred in colloquial Korean) I'm into 오빠. I like 오빠

걸 is from 것을 .. and I've asked it a few weeks ago.. kkkk check my profile! Lol

나는 오빠가 좋은걸 sounds like 나는 오빠가 좋은걸 (어떡해? what should/could I do about it?) Such subtle emotion is hard to capture and express in words...