You aren't performing an action on the plate. That would make the plate Akkusativ case. You're just equating the plate with something else: "Es ist ein Teller."
You can think of the verb "sein" as an equals sign. The words on either side both stay Nominativ.
Es ist ein Teller
Es = ein Teller
Of course not, there is a first time for everything. Did you hover? That's often the first time we see a word. If you had it on a listening , yeah that is tough. But next time... The Duo system takes some getting used to. I'm surprised you've reached level 8 and haven't noticed that things are done differently around here. Don't worry it took us all some time to get used to but after that you know what to expect or not to expect. Relax, enjoy. Hearts will be lost and gained. It's all in the game well sort of game. Here's a lingot.
Here you go happy to help. http://www.duolingo.com/comment/1352379
and another: http://www.duolingo.com/comment/1278938
These are created by other learners for our benefit.
At the bottom of this page you'll find the official Duo Guidelines. Be sure to read those also. What ever you need to ask come to the Comments and someone will be able to help. Good luck, Enjoy. Here's a lingot for you efforts.
The word-by-word translation isn't the best way to learn a language. For example, in Bulgarian I'd always use the equal word for "with" in this case, and never the equal word for "of", it wouldn't make sence (it'd mean "a plate MADE OF rice", lol). I'm still not sure if it is correct to say "a plate with rice" in English, though it sounds better to me (help, please - would this be correct?). Every language is unique.