"January is a very cold month."
Translation:Leden je velmi studený měsíc.
My answer - Leden je hodně studený měsíc - was not accepted. The correct answer offered was "Leden je hodně studeným měsícem." This looks like it uses the instrumental case, which I haven't come across yet.
Since the correct answer above uses "velmi" instead of "hodně," did "hodně" make my answer wrong? (I've reported it, in case my answer is actually okay, since it could be difficult to find the exercise again later...)