"They forget that they are in a museum."
Translation:Oni zapominają, że są w muzeum.
Is the 'oni' at the beginning required, or would the sentence work (out of context) without it?
It works fine. It always needs context, you need to know who "oni" or "one" are. In English too I believe. I do not know if it works on Duolingo though.
The same question. "Zapominają że są w muzeum" is not accepted here, although the same omission is accepted in other examples.
You have two clauses here: "They forget" and "they are in a museum". If the subject of the second clause is the same as in the first one, it sounds really strange to put it explicitly. This "one" just sounds weird. Why say it, if it was already obvious from "zapominają"?
Yeah, technically it could be "They (A and B) forget that they (C and D) are in a museum", but that's rather unlikely and it doesn't help learning.