"You see the coffee."
Translation:Du siehst den Kaffee.
There are certain cases in German which alter the gender of nouns. The accusative case is one such example, and in the accusative case the gender 'der' changes to 'den'. Whenever a noun is the 'direct object' of a sentence the accusative case applies, and so because the apple is having a verb done to it 'the apple is being seen' this is the answer.
I don't know if this is still useful and maybe my methods are a bit over-complicated, but they work for me. if a german word ends with an E it is "die". When it is "the" the "die" becomes a "den" in most cases (this is not all cases so don't take my word for it everytime, "zeitung" is always "die" for example.
Newspaper (Zeitung) is a feminine Noun, that's why it is always 'die' because it is for Feminine Nouns. You also use "die " when you are dealing with plural Nouns like "Erdbeeren" and "Ente". Den is used for Masculine Nouns that are being acted upon. In this case, Du siehst den Kaffee, the coffee is being looked at. Hence, you use "den" when it is a masculine Noun and an action is acting on it.