I know the man. You are talking about him.
이걸 I know the man about whom you are talking.
이렇게바꿀때요 about whom도 생략 가능한가요?

I.ll do anything . I can do that for you.
이거 what 이용해 바꿀때요.
I.ll do what I can do for you.
맞나요!