"She eats the apple."
Translation:Sie isst den Apfel.
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The accusative case is used when the article applies to the direct object of the sentence. So when you say, "She is eating the apple", the apple is the direct object and so the article (the) will be in the accusative case. I found this web page http://german.speak7.com/german_articles.htm and it REALLY helped me understand what's going on. It'll take just five or eight minutes to read!
You are asked here to translate "She eats the apple", It's "Sie" for She, "isst" for eats (because the conjugation of the verb Essen for He and She is "isst" and not esse, which is used for "Ich esse" - "I eat"), and then "den Apfel" for "the apple". It's not "Eine" because eine is the word "a / an" for feminine, and even not "der" because they explained in the hints that when there is a masculine noun in the accusative form, it should be changed to "den" instead of "der". Hope I was clear enough, because English is not my native language.