"¿Nos ayudas a limpiar el baño?"
Translation:Can you help us clean the bathroom?
It rejected "would you help us clean the bathroom". Neither "would" nor "could" nor "can" is literal. Literally, it means "do you help us clean the bathroom" or "are you helping us clean the bathroom". But it is being used in the sense of asking for help. In that case "would", "could" and "can" are equally acceptable, indirection being a polite way of expressing an imperative.
Very much agree with you with regard to these exercises where "can" is the only English version accepted. That simply is not the only valid translation.
To try to get around Duo's requirement, I tried an even simpler translation, at least, simpler in the sense of using the fewest words: "Help us clean the bathroom?" In this case the "would you/will you" part is implied. Yet that answer was ALSO rejected (I reported that it should be accepted), and it supplied the completely unnatural "You help us clean the bathroom?" as the correct answer (which I reported as unnatural).
I agree that there are several ways of making a polite request in English, and all of them are as acceptable as using "can" (some of them arguably more polite). I also think the literal translation of "are you helping us clean the bathroom" should be accepted, too (given the absence of more specific context).
"Will you help us clean the bathroom?" accepted as at 11-Sep-2018.
I think 'Would' is probably used more often these days. 'Can' often gets a retort that "I can (i.e. know how to or am physically able to) and that's the answer to your --- question".
I try to lessen the fear or distaste of getting it 'wrong' by DuoEnglish and sometimes deliberately type what I think is a good expression (as a contribution) then hope that DL will eventually assess its merit.
These "implied" additions of "will", "could" and "can" bother me. It makes me wonder how one reliably intuits when and when not to add them.
For the pedants among us, the use of "can/could you help us" opens oneself to the risk of the sarcastic response, "yes I can/ could help you - but I'm not going to!" Would is a safer often.
I just got that response, too. I put "Will you help us clean the bath?" DL rejected it and gave the correct answer as "Will you help us clean the lav?" We never use that expression any more, it's really dated. It would probably be better to say "Will you help us clean the 'bog'"! Common, but equivalent to 'lav' in current terms.