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Well, German distints four cases: nominative (der-die-das and die for plural), accusative (den-die-das+die) and later on dative and genitive. Now we only use nominative and accusative. Make sure you apply the -en ending at masculine words in singular, accusative case. For further info search for 'German declension'
Nominative for subjects and acusative for direct objects. "Der apfel ist rot" (The apple is red) where apfel is nominativ uses der. But "Der Junge isst den Apfel" (The boy eats the apple) apfel is acusative because it is the direct object. The thing the boy eats. You have to ask the question "Was isst der Junge? Den Apfel" and this is how you know it is akkusativ and you use den instead of der (the two of them being masculin)
Hope it helped! :)