Translation:I have not finished the homework yet.
If you think your translation is valid, report it and they might add it. They don't have every possible variation in their answer list, and the more you stray from the most direct translation, the higher the chance that it won't recognise your answer
Basically you're better going for the safest, most boring answer, just so you can prove you understood it. Over time it'll recognise more natural translations, but this is a pretty new course! Just a warning
At least you don't lose hearts and fail anymore. Having to restart a lesson was not a good time!
There's no present perfect in Japanese, so using an English present perfect conjugation is more based on context. Japanese students are typically taught to use the English present perfect when:
1) Talking about experiences
Igirisu ni itta koto ga arimasu.
I have been to England.
2) Talking about something they started doing in the past and still do now.
7nenkan, Hokkaido ni sunde imasu.
I have lived in Hokkaido for 7 years.
3) When using the adverbs already, yet, and just, like in this sentence.
There's a pretty long explanation for this, but I'll sum up it up in the best way I can. は and が are both topic indicators, right? In english, it's pretty hard to not see the difference between the two, but は is a topic particle for specific topics, and が is for unknown topics. That's only one part of it, however, and if I wanted to explain it all, I'd have to send you like 10,000 words on why. I strongly suggest going to http://www.guidetojapanese.org/blog/2005/02/05/the-difference-between-and/ . Read this blog entry, it makes a lot of sense and is way more useful than what I just said.
Well; が does not indicate topic, but subject (who or what performs the action). In a lot of cases the topic overlaps with the subject (because usually the subject is what/who we are talking about), so we use は instead of が, but that does not mean that they have the same function.
は doesn't really have a syntactic function (it does not indicate "what function" the element has in the sentence or how it relates to the verb), but just kinda sets the context of the sentence. A little like a hashtag before the sentence would work providing the topic (like you would tweet: "#me doing homework" or "#now doing homework").
Though that, it's true that in some cases where the subject is also the topic, which particle you usually use is totally phrasal