Du vs. le
I don't understand why it is "Je mange du citron" and why it'd not "je mange le citron." Same thing vice versa.
I understand that "du" means "of" and "le" means "the."
"Du" means "of the", as a contraction of "de" and "le", but it could be better translated in this context as meaning "some", as in "I eat some fish" vs. "I eat the fish". Essentially, they mean separate things.
I have this problem and I'm not new to French (though my rating on duolingo might argue that). I was a French minor in college. I'm finding a lot of problems with the ambiguity as far if something is meant to be plural vs singular by vague English sentence structure. Because colloquially, we know what someone means in English by the context. But a single vague sentence with no articles or pluralization (or a work that is naturally plural and singular at once) is almost impossible to get correct without conversational context.
Also, I find the voice for the spoken sections to be very difficult to distinguish some articles apart, and I practice with native speakers -they are way easier to understand.
Part of any struggle might be lack of understand, part of is might be duolingo, and part of it is lack of context for the sentence.