Should "les ours mangent des citrons", here, be translated as "the bears are eating lemons"? Whereas "des ours mangent des citrons" would translate as "bears are eating lemons". The former refers to a specific groups of bears and the latter refers to bears in general, or some bears.
At least accepted "the bears eat lemon", but now that I think it over, it could have been rejected.
"bears eat lemon" sounds better than "bears are eating lemon", since "are eating" is like indicating a particular group, not all bears in the world are eating lemon (just now or at the same time), Instead "eat" gives a sense that the speaker thinks that bears usually eat lemon, as a habit, kind of "bears eat berries".
And, for generality or in a sense including all members of a group denoted by a word, they say that definite articles are used too.
Duolingo seems pretty relaxed about "les" vs "des" (and "le" vs "de") at the beginnings of sentences. This isn't the first time it's been flexible about having or not having a "the" in the English counterpart. I don't know whether this is an artifact of their software or a true property of French.
This has nothing to do with the software since all translations are entered by hand by real people.
Generalizations need a definite article. But specific subjects too.
- "Les ours mangent des citrons" looks and feels like a generalization, like "les vaches mangent de l'herbe"; but in reality, bears do not eat lemons because there are few if any lemon trees where bears live.
My feeling is that the best (sensible) translation should be "The bears eat/are eating lemons" because it is possible in specific situations.
However, both "bears" and "the bears" are accepted for the sake of consistency with other sentences and generalizations.
At last, I don't understand your comment: about "les" vs "des" (and "le" vs "de") at the beginnings of sentences. At best, I can tell you that the indefinite "des" never translate to "the".
To me, in the audio version, both 'ours' and 'citron' are pronounced very strangely. When this happens, [us humans] try to find a logical match but this sentence is unlikely to arise in everyday french life! Come on Duolingo - you are great but - please stop inventing nonsensical sentences just to include words that you want to test - please. Rant over!
Yes, it is called a liaison. http://french.about.com/library/pronunciation/bl-liaisons-f.htm
No, you can't use "de citron". Let's back it up a little. "Un citron" = a lemon. "Des citrons" = lemons, or (some) lemons. "Les citrons" = the lemons. English does not have a specific counterpart to the French "des" (meaning the plural of "un" or "une"). If it helps the expression in English, you may use "some" when you are only referring to more than one, but it is nearly always ignored in English. English nouns can easily be distinguished in spelling and by hearing. French nouns often sound exactly the same in singular or plural form so in French, the "des" is very important. http://french.about.com/od/grammar/fl/Du-De-La-Deshellip-Expressing-Unspecified-Quantities-In-French.htm