Which is fine. It is about being understood. You are never going to get fluent from a course. Language changes, and even native speakers are learning new things.

That's besides the point. My point is "definiteness" is not a concrete concept in Turkish and if you're academically analyzing a sentence you'd have to use other types of information (accusative case, the word 'bir', context, etc...) to judge if a noun is definite or indefinite (or maybe something in the middle?). But Duolingo teaches as if this distinction is as strict in Turkish as it is in English. In English "a bear" and "the bear" are different things even without context whereas in Turkish "ayi" [bear] and "bir ayi" [one bear] mean more or less the same thing outside of a sentence and I'd just use "bear", but in some contexts they'll mean different things. "Ayi cikabilir" roughly means "caution, bear in area" and as you see you don't use "bir" but it is still indefinite. Duolingo would reject this answer. "Bir ayi cikabilir" would be Duolingo's answer, but it sounds awkward, no one would say that.

And there are no hard rules. In that sentence you can't just put +I to make it definite as I described above "ayiyi cikabilir" << this sentence doesn't mean anything. I think this is something your brain has to pick up implicitly while you're speaking/listening, I'm not sure if someone can discretely provide you a set of rules that always works. Just like in German you cannot translate "the" > "der". This is like Duolingo saying 'the'='der' and rejecting 'das' and 'die'.

So, it is not fine, it is wrong. And the wrong thing is not that Duolingo has a bug, it is that it has made-up grammar rules, which users still have to learn. If they never learned this rule, they'd be as well understood.

(I know you were replying to commenter discussing French, but I'm guessing something similar is going on)

Sure but my point was that Duolingo basing all of its courses on translating from one language to the other seriously limits its ability to actually teach you a language. My native language isn't English but as I'm writing these words I'm not translating, I think in English directly. If I switch to "translating" mode I'm a lot slower and I end up with unidiomatic English.

Any decent language learning course should try to immerse you in the target language as soon as possible, forcing you to actually think in the language instead of your own. There are many ways to do this, for instance making you answer questions. Instead of telling you "translate 'the cat is black' in French" they'd show you a picture of a black cat and ask you "Quelle est la couleur du chat?" and you'd have to answer "Le chat est noir". No English involved, like in real life. The problem of course is that such an approach is hard to correct by a dumb algorithm, especially as the concepts being taught become harder.

I disagree. The nuances that change are on a different level. I have learned a number of languages from books and this is taught pretty easily. If you can't conjugate a verb, you don't know much about that language.