First things first.
I learnt the explanation for the below from the following Youtube video: Understand the GERMAN CASES - Accusative, Dative, Nominative, Genitive and the book German for Dummies

In German, the position of words can be interchanged while still retaining the same meaning.
For eg. Der Hund beißt den Mann can be written as Den Mann beißt der Hund. Both mean the same (The dog bites the man), unlike english, where the literal translation becomes:

Der Hund beißt den Mann = The dog bites the man.
Den Mann beißt der Hund = The man bites the dog.

So the reason for this difference is because in German we have different cases, which modifies the articles (and other stuff) according to the function. But in English the definitive article is only ‘The’. So here the ‘The’ associated with ‘Hund’ is Der and ‘The’ associated with ‘Mann’ is ‘Den’, which denote different functions as we’ll see next. The 4 cases in German are Nominativ, Dativ, Accusativ and Genitiv. I’ll explain each one below.

  1. Nominative Case: The subject of the sentence, i.e the one who is doing the action will always be in nominative case.
    Eg. Der Junge stiehlt eine Wurst (The boy steals a sausage) : Here the action is done by the boy. So the article related to Junge is in Nominative case - Der.
  2. Accusative Case: The direct object of the sentence, i.e the one which is directly affected by an action will be in Accusative case.
    Eg. Der Junge stiehlt eine Wurst (The boy steals a sausage) : Here the action is done on the Wurst. So Wurst becomes the direct object, and hence it is in accusative with article eine
  3. Dative Case: The indirect object of the sentence, i.e, the object for/ to which the action has been done by a subject on another object will be in Dative case.
    Eg. Der Junge gibt dem Hund die Wurst (The boy gives the sausage to the dog) : Here the action is performed by the boy on the sausage for the Hund. So, Hund becomes the indirect object and is in Dativ case with article dem.
  4. Genitive Case: The Genitive case is used to denote possession. The one which possesses something will be in genitive case. Eg. Der Hund des Jungen (The boy’s dog) : Here the boy possesses the dog. So the article (again, can be other stuff as well) related to Jungen is in Genitive case with article des