"The young man lives in New York."
Translation:Iuvenis Novi Eboraci habitat.
They have explained it here: https://www.duolingo.com/skill/la/places/tips-and-notes It's a good idea to read these before starting a new skill. I actually keep them open in a separate tab.
Ending in -e would be ablative in the consonantic declination. Ending in -i in this case is the locative.
Just to be awkward I wrote Eboraci Novi, which strictly speaking should be just as correct as Novi Eboraci. It was considered a mistake, and I’m wondering whether it should be. When does a place name in Latin become fossilized into one particular order?