There are 4 cases of article form. Few days ago I have written about Nominative. Today I would like to introduce another fellow call “Akkusativ”. Akkusativ is a case of Noun working as an object of the sentence. Of course there will be some changing to the article when the case is change from Nominativ to anything. This time is also the same. When the sentence have akkusativ, article Der is changed to Den. Article Die/Das are still using the same form.
For example, I buy a/the table “Ich kaufe den/einen Tisch”. So, table is the object of this sentence. Have you noticed “der Tisch” is changed to “den Tisch”?
Please see the table below for the article form of each cases.

Nominative	Der Tisch	Die Maus	Das Telefon
Dativ	Dem Tisch	Der Maus	Dam Telefon
Akkusativ	Den Tisch	Die Maus	Das Telefon

Note: I found that a free poster from DeutschAkademie called “Kleine Grammatiktafel der DeutschAkademie” is really useful to me. It is helping me whenever I write a sentence. Please take one home ^_^.