"On Monday we learn."
Translation:Am Montag lernen wir.
In German, usually subject goes in first place than verb and so on, so the right translation should be "Wir lernen am Montag"
Subject first is the default, unmarked word order.
But a number of things can be placed first in the sentence to topicalise or emphasise them, including adverbials of time such as am Montag.
The original answer Duolingo gave was Am Montag werden wir lernen. Werden means to become. Is this an idiom, and are there any notes available about it?