"The juice is bad."
Translation:Der Saft ist schlecht.
I usually attribute "der, das, or die" to feminine, masculine, or neuter. In the tips for the Accusative Case, there is a chart that lists the different ones for the three categories. So in summation, identifying which one you use isn't the difficult part, the more difficult part is identifying masculine, feminine, or neuter, which unfortunately, Duo does not extensively teach us. So my suggestion is to make flashcards or find some way to effectively pinpoint German nouns (like make up a character or personality for each word).
In German when we say something is "bad" we usually mean that it is in some way defective of it's original design. So for example a "bad" television is a broken one. However, if the same television were being described as "not good" having the context of being "good" means it still works but we should probably buy a new one. So for this example "bad" juice is juice you won't drink, "not good" juice is juice you would drink but not prefer.
Actually, you might hear "Der Saft ist nicht gut" in many situations where people are trying to convey that the juice is bad. Often, the indirect, reserved/polite statement may be preferred to a plain "Der Saft ist schlecht!" However, this is a cultural, not a linguistic issue, which DL probably cannot make individual allowance of in a language course.