"She is a bad person."
Translation:Ona jest złym człowiekiem.
How can I know when a masculine noun, in instrumental case, takes the "-iem" ending versus the "-em" ending?
Why is "człowiekiem" used for "person" and not "osobą"? If "człowiekiem" is usually used for 'human", and "osobą" is usually used for "person", how does one know when to use the other?
Well, they do mean exactly what you wrote, but the issue is more complicated. Generally, "osoba" seems to be used a lot less often in Polish than "person" is in English. I wrote everything that came to my mind about this issue here: https://www.duolingo.com/comment/19428542
But actually, it suits this sentence perfectly okay and is accepted: "Ona jest złą osobą".