"I already finished my homework."
it would make far more sense to have the English prompt say "i already did my homework" as opposed to "finished" since やる is the verb they want.
Shouldn't the particle in this sentence be を not は (as in, 宿題をやりました) considering that homework is the object and I is the subject?
I think so too. But I'm not skilled enough to know for sure, because it could be like ''As for homework, it is already done.'' Or some meaning like that which could make ''homework'' the subject.
I put わたしのしゅくだいをもうやりました and it marked incorrect, saying the correct answer should be わたしはしゅくだいをもうやりました。Reported, 3/6/18
No, 終わる is an intransitive verb, so that would mean "the homework has ended already" (also, no -te imasu form). Maybe you could use it as in 宿題はもうやり終わりました "I already finished/stopped doing my homework", but the easiest is to stick to Duo's suggestion.
I think you can use おわっています for closed, as in a shop which is and continues to be closed. Wasn't that in one of the exercises?
やめてました could be correct right? Any japanese native kind enough to teach me please?
"I already finished my homework" would use 終わる（おわる）, whereas "I already did my homework" would use やる.