"We are women and they are men."
Translation:אנחנו נשים והם גברים.
This is marking אני נשים והם גברים״" as correct. Can "אני" be used in place of "we" as well?
not אני but אנו, which is (as far as I look up just now) an archaic/formal form of אנחנו.
It's either archaic or formal. And while most people don't have a use for archaic forms, they do for formal. Even if you never say אנו, you will defenintely hear and read it, and should be able to understand it. As for mentioning it, from what I understand the program isn't able to make a destinction within the lessons. But they do usually mention such things in the Tips and Notes on the lessons.