"What is it, garlic?"
I put it "cos'è, l'aglio?" since typically in romance languages you use the definite article a lot, but it marked it wrong. Is that just an oversight of the system, or is there a grammatical reason why it wouldn't work?
Italian does use the definite article in many situations where English doesn't. Most of those times it's actually more correct to use the definite article than do an word-by-word translation from English... But in this case I don't know enough to be sure either way.
This is the most useful link about the definite article I've found (below) and while it still doesn't give me enough to be sure, I'm going to guess that you don't use the article because in this case you're not referring to garlic in the general collective sense or as a category. (Instead you are pointing at a particular object and saying "what is it?")
But if a native speaker happens to weigh in on this discussion, take their word over mine...
I just gave the same response (5/20/14), and it was accepted. So it seems like people must have reported it and the system was updated.
I don't know how understand this sentence because I'm not native English speaker or Italian speaker. With comma it looks like someone speaks with garlic about some other thing and asks "What is it?"
Yes, it does appear as if we are addressing the garlic. I believe duolingo's intentions would have been clearer if it had been written "What is it? Garlic?"
Think of it like this: Cos'e? Aglio? And you will understand it. Sorry, can't type the accent.
when do we use qual and when do we use quale and when should we use cos' and when cosa
That's exactly that. Used in other languages too. You avoid having your mouth open too long ...