What is the difference between Die and Den
How come sometimes they use "die Apfel" but other times they use "Den Apfel"? What is the difference, how do you tell them apart and how do you use it?
You mean "der Apfel" vs "den Apfel" ("Apfel" is masculine, therefore it takes the masculine definite article "der"). You use "der" for the nominative case (you can think of it like, "the noun is doing something"), e.g. Der Apfel ist gut. (The apple is good.) You use "den" for the accusative case (you can think of that like, "the noun is having something done to it") e.g. Ich esse den Apfel. (I am eating the apple.) For the feminine (die) and neutral (das) articles, there is no change between the nominative and accusative forms. This is hard for us English-speakers to understand at first because in English there is no difference in form between the nominative and accusative cases.