What is the difference between "to nie" and "nie jest"?
not much difference but to nie/to nie jest/nie jest to need noun in nominative, and nie jest needs instrumental.
I reccomend reading tips and notes - defining https://www.duolingo.com/skill/pl/Defining
Thank you! So I should have reported that my translation "kot to nie człowiek" was valid, right?
Yes , that answer is good.
Lots of people with fur babies just got offended.