뭔가=something *뭔가 이상해! or ? = Something wrong! or ? 뭐가=what *뭐가 이상해? = what's wrong? (It's used only when you ask) 뭐지=What's it or that or this or etc? =(그게, 저게, 이게) 뭐지? *뭐지 is a asking sentence. The subject tend to be omitted in Korean. 뭔지=What something is *뭔지 is a phrase. 뭔지 모르겠다 = I don't know what it is. FYI, There are omitting too. (나는) (그게)뭔지 모르겠다. 왠지=guessing without specific reason ----->I can't figure a suitable term, word. Sorry 틀린곳 있을지도 모르니 고쳐주세요~~^^