The Accusative Case

The accusative case is mainly for direct objects and objects of accusative prepositions. A direct object is the noun or pronoun that receives the action of the verb (i.e. they are acted upon).

Example: 
Die Frau füttert den Hund. / The woman feeds the dog.

The direct object can be identified by asking, “Who or what is being ‘verbed’?” For the above example we can ask: Who or what is being fed? The answer is "the dog". Hence, the dog is the direct object of the sentence. It is in the accusative case. 
The accusative case only affects masculine nouns: the definite article changes from “der” to “den.” The indefinite article “ein” (masculine) changes to “einen.” Feminine and neutral nouns do not change.



Examples:   
Ich rieche den Kuchen. I smell the cake. 
("der Kuchen" is a masculine noun. In the accusative, "der" becomes "den".)

Ich rieche die Blume. I smell the flower
("die Blume" is a feminine noun. In the accusative it does not change.)