The issue of Idiomatic expressions has been raised by other users (http://duolingo.com/#/comment/5378 – http://duolingo.com/#/comment/12343 – http://duolingo.com/#/comment/98067) but I'm creating a new discussion, because two of them are many months old, and the third has a misleading title.
That said, I've encountered this in a Portuguese lesson: "Para ela, é oito ou oitenta." Translation: For her, it is eight or eighty. (Real translation: For her, it's either black or white.)
This kind of literal translation is unacceptable. But I do think idiomatic expressions are a vital part of everyday language comprehension. My opinion is that it should get its own lesson "binder", probably as an advanced subject, lower in the skill tree.
In my experience, although i can't think of an example when the difference between idiomatic and litteral translation is as big as your example in Portuguese, your better answer being marked as a mistake is not the final state of that question. the proper thing to do is to flag a problem with the question, select "My answer should be accepted", and go on. After duolingo fixes it, users will get it flagged right answer with the idiomatic translation. If the use litteral translation, they get "Correct. Here's another correct answer: idiomatic expression", or something along these lines. The curious learner will always read these and immediately realize that is an idiomatic expression... It should be highlighted in a heavier fashion, that I agree with.
For your particular example in Portuguese, I really believe it is to be considered as a mistake/bug. Portuguese is in beta! German has been out of beta for months and you still get stuff of this sort in later lessons, so really it is not surprising that you find imperfections in the portuguese skill tree. Just flag them, help duolingo shine their awesomeness all over it and i'll send you love when i learn Portuguese in 2015! Good karma!
Wow sorry if i sounded that way, it really wasn't my intent!
Your stance that litteral translations shouldn't be accepted is very legit. I don't know where i stand on that. They definitely should phase out as the student advances, but rejecting them all together sounds a bit harsh to me. It might be frustrating and discouraging to the user, and therefore counterproductive. Idiomatic expressions are possibly the most fun part in the mostly tedious process of learning a language, but they are often very obscure to people of a different culture. They also don't always have an equivalent in the other language, or only an approximate one, etc. Of course you can argue that one can narrow it down to the more obvious cases, like the one you cite, but then you'll have to draw a line and that's never easy.
Also, I don't believe litteral translations are meaningless. They are the key to understand the metaphore in the first place, and imo should be known as well as an idiomatic one. In your example, it took you 2 sentences to explain the expression in Portuguese, now i know it, know what it means litteraly, know what meaning people will actually use it for, and understand the link between litteral and idiomatic meaning. My belief is that i would have got the exact same knowledge if i had translated the expression litteraly, got duolingo give me a green/red feedback (like right now in case of typos) telling me "what you said's alright, but really it means idiomatic expression". I'd prefer that over being promted an error and the idiomatic expression ; of course it is subjective.
I'd probably be up for treating the litteral translation as an error after N encounters though. After all, there is already a system to keep track with one's familiarity with particular words, it should be possible with phrases too...
No need to apologize. Besides, you have a good point, one that eluded me because for me the example given was obviously clear as water, since I'm Portuguese and fluent in English, which is that Duolingo should tell you what the literal translation is, for you to truly understand why the expression bears such meaning. But I still think you should be taught both (literal and real) and then only expected to give the proper translation back.
This is where the problem lies. Beginners don't have any way to judge whether something is idiomatic or not and the way it's done at present still doesn't actually tell you whether it is idiomatic or just a translation error. You are just marked incorrect. I like the idea of niaugel's suggestion of an explanation to resolve this but I'd also be happier with being given the opportunity to learn idioms before they're flung at us.
Don't get me wrong. I'm loving Duolingo. I wouldn't still be doing it nearly six months on if I didn't but there are a few instances like this where some relatively minor changes would remove something irritating that doesn't need to be there.
I completely agree with you guys. Here is the comment I put in a discussion started on a german idiomatic expression: "I don't agree to put idiomatic constructions in these exercises, since it's almost impossible to guess right without knowing them in advance and the first objective of the test should be to teach grammar first. If the plain translation is not acceptable, Duolingo should explain why is not in the correction, saying explicitly if it would sound weird in a conversation or in a specific context. Then I would propose that idiomatic constructions are somehow fixed in number during the units, or better to dedicate a single unit in the whole chapter to improve the skill to detect and use them. I would also propose that Duolingo gives the student the possibility to review at any time all idiomatic constructions encountered during the learning process, like for vocabulary list."
I agree with aucunLien that additional feedback, similar to that for typos and accents, is the right response: If I translate an idiom literally, it should be marked right (because people hate losing hearts), but the text should say "Nice try, but actually, you would say [idiom goes here]". Otherwise, people are either losing hearts "unfairly" or learning the language wrong.