"I have a glass of water."
Translation:Tôi có một ly nước.
Duolingo sometimes doesn't accept sentences that are not properly accented, I guess.
It sounds unnatural. I think in this case we are talking about the water in the glass rather than the glass itself, so we should not put cái before.
Same reasoning for "đĩa cơm" ? Your explanation cleared up my confusion. Many thanks.