"I am not wearing a shirt."
Translation:Nie noszę koszuli.
I'm glad to see this in this exercise the Duolingo authors are taking it one step at a time, introducing only the fem. sing. genitive, so avoiding even more confusion. Hopefully the next lesson will take one of the other genders only. A lot of people are learning by example only, I presume.
So in negated sentence you can't use the Instrumental case. Am I right?