Note that the only words that change their form in the accusative case are the masculine words, ie the der words. All the other forms are the same as the nominative case.
It is a theory of the Government and Binding framework and its successors. There are two Cases that are generally recognized in English--the nominative Case and the accusative Case--the latter is also ...
If dual-case prepositions describe movement from one place to another, they are followed by the accusative case. Otherwise, they take the dative. Ich stecke die Schuhe unter das Bett.
In total there are four cases in German: nominative, accusative, dative and genitive (some claim that the genitive is fading out, but I am clasping onto it firmly with both hands, for it is the German ...