I wrote "You are not stupid, this problem is hard.", which I believe is correct and even more common than the translation Duolingo provides. Am I mistaken?
That's correct for a sentence like this - ты не глупый, этА задача трудная.
"You're not stupid; it is a difficult problem" should be accepted, Duo.
I thought so too, but would that not be "трудная задача"?
If we're asked to transcribe this from audio, and эта sounds indistinguishable and makes a correct sentence, it should probably be accepted. (German has made similar accommodations for "ist" and "isst", for example)
I heard 'эта' here, which sounds more or less the same as 'это' in Russian; could this also be correct?
They sound the same, right.
In fact, you can use "эта" here, but the sentence will differ grammatically. It will go as "this problem is hard", rather than "it is the problem that is hard".
"ты не глупый, эта задача трудная" was marked wrong. If это and эта do sound the same and this sentence also makes sense, I think it should be accepted.
Shouldn't it be трудна? I thought when you're using the form "[thing] is [adjective]", the adjective takes a short ending.
What is the difference between задача and проблема? Is it that a проблема is like the stove is on fire, while a задача is something you get on an exam paper?