"Although I want to, I cannot."
Translation:Apesar de eu querer, eu não consigo.
"Eu querer" is in the Personal Infinitive (Infinitivo Pessoal) which can function as a kind of simple replacement for the Subjunctive case (as it does here). I doubt that Duolingo should be using this here before it is introduced in the lessons. Here is some further information and reading.
I can't find anywhere in the Duolingo Course where they specifically address the personal infinitive. I've already completed the course so I can look at all the lessons, and I don't see the personal infinitive anywhere Rather than cover it in depth, I think they've just thrown in a few examples here in "infinitives 2" to let people figure it out on their own. To help you figure it out on your own, Chunga posted some pretty good links.
In addition to the links about personal infinitives posted above by Chunga, I also liked http://www.learn-portuguese-with-rafa.com/portuguese-personal-infinitive.html
As Chunga states, the personal infinitive can take the place of the subjunctive. If you've ever studied a language with subjunctive, you may remember something about subjunctive triggers. Just like the foreign language textbooks that list subjunctive triggers, Rafa of Portuguese with Rafa, lists some "personal infinitive triggers." Some of the "personal infinitive triggers" are the same as subjunctive triggers. Among them is "apesar de."
So we have:
apesar de eu querer… = even though I want to…
apesar de tu quereres… = even though you want to…
apesar de ele querer… = even though he wants to…
apesar de nós querermos… = even though we want to…
apesar de eles quererem… = even though they want to…
I'm still learning this personal infinitive thing; it's still new and strange to me. So if someone comes along and says something different from what I've said, by all means believe them instead of me.
Other correct answers?
Duo also accepted subjunctive. I answered "Mesmo que eu queira…" and duo accepted it.
My answer: "Embora eu quero, não posso." was marked wrong, but one of the suggested answers is "Embora eu queira, não posso.", which seems to be a "subjunctive" form of the verb (not that I know what exactly that means).
Is my answer really unacceptable, and if so, can someone explain the difference between these two?
Here is my 2 cents. The situation suggests one thing certain (I cannot) but the why is open to interpretation (whether I really wanted or just an excuse - doesn't really matter - I still can't). So one could choose a number of tools here to imply that uncertainty: Subjunctive (Conjunctive), Conditional, or just past imperfect as a common way (eu queria -more informal) to replace the Conditional (eu quereria - DL might/should accept this too but I haven't tried)