"We have a big piece of cake."
Translation:Noi avem o bucată mare de tort.
I tried to translate this sentence as "noi avem o bucată mare tortului" and got dinged for using "tortului" instead of "de tort."
What sort of rule is being violated here? From other languages, I might expect that the genitive can serve a partitive function, and I don't think I've seen anywhere in this course rules for how to decide between "de XXX" and "XXX[gen]."
My instinct is that my phrasing is legal but just not preferred for Reasons, but I'd love to hear if someone can explain why my phrasing is bad.
Noi avem o bucată mare a tortului would be correct and closest to your desired usage.