Why is "auf" using the accusative case here?
It's in the accusative because motion is implied.
Very useful! Thanks!
Why is Teller here translated as plural?
Because there's no article.
thanks Christian I was about to ask that
It could also be "You put plates on the table"?
That was my response and it got rejected.
Yes, it could. Is it accepted by duolingo?
I guess it wouldn't be accepted because "stellen" determines the plural (they)
No. "Sie stellen" can both be "They set" or "You (formal) set". The verb doesn't change. If "Sie" begins with a capital letter it's you (formal) and in this case "Sie" is at the beginning of the sentence, so we can't know without any context.
What about She sets plates on the table? You use a different form of stellen?
Yes that's right. I'm not really sure, which one (I haven't learnt all forms of stellen), but I would guess that should be "Sie stellt Teller auf den Tisch."
I guess it should be acceptable to translate the sentence with both "plate" and "plates ", because plural form it is not clearly indicated.
No, it can only be plural as there's no article.