The accusative case corresponds to direct objects.  Here are the accusative forms of the definite and indefinite articles.  Note that only the masculine changes in this case.

Note:  Some masculine nouns add an -(e)n to the accusative form, such as international nouns ending in -t (Dirigent, Komponist, Patient, Polizist, Soldat, Student, Tourist, Journalist); nouns ending in -e denoting male persons or animals (Drache, Junge, Kunde, Löwe, Neffe, Riese, Vorfahre, Zeuge); and the following nouns: Elefant, Herr, Mensch, Nachbar.  And wen (whom) is the accusative of wer (who).

German uses the case system to show the function of a word in a sentence, whereas English relies mainly on word order.  Take, for example, the following sentences:  Ich esse den Apfel translates into I eat the apple.  In German, you can switch the word order around without affecting the meaning.  Den Apfel esse ich is also I eat the apple, but in English, if you were to change word order, you would have to say the apple eats me.  English does not accomodate for the direct object to be placed before the subject and verb like German does.