wow... I also need a professional korean teacher in order to explain that.. anyone? First of all, I personally like that song. As you know, as other lyrics, words are used implicatively. 1) when we were young we did not realise our youth when we were young=우리가 어렸던 시절에=>젊은 날에는=>젊은 날엔 we did not realise our youth=우리는 우리의 젊음을 알지 못하고=>젊음을 모르고 _why we added 고 to 모르다 ? what's the role of it ? 모르고 = 모르다 + and(그리고). Probably the sentence was not finished. _ what's the role of 음 in 젊음을 ? when you transform verb to noun, we use it. _I read un other lyrics they said 모르지만 sometimes we said 모르지만 other time 모르고 and also there's 몰라 and 모르껬어요... what's the difference? I never curious on this before, but now i am. 모르지만 is mostly used between the sentences. 모르다+but(하지만)=모르지만 모르고 I dont really use it.. 모르다+and(그리고)=모르고 몰라 is mostly used. It is more like the conversation version of 모르다. Be aware that it can have various expression by the tone of the voice: annoyingness, lacking sincerity.. 모르겠어 polite expression of 모르다 with sincerity RIght, guys?