"Warsaw is a European city."
Translation:Warszawa to europejskie miasto.
In most situations, both "to" and "jest" are perfectly fine. You just have to remember that after "to" comes Nominative, and after "jest" Instrumental.
"to jest" is also correct, I guess, but to me it seems a bit strange and rather unusual. I think we generally don't accept it unless someone asks specifically.
You can take a look here: https://www.duolingo.com/comment/16373167