"They forget that they are in a museum."
Translation:Oni zapominają, że są w muzeum.
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.