What case should be used where in a sentence?

German, unlike many other languages, has 4 cases (nominative, accusative, dative, and genitive), each with their own version of the article (der, die, das, dem, den etc)... but what fits where? There are two main ways to identify which cases to use, and here I will explain the firat main way:1) Identifying different parts of the sentence.eg1. The man is writing a letter to his wife. Subject (nominative): The manDirect object (accusative): a letterIndirect object (dative): his wife.DER Mann schreibt SEINER Frau EINEN Breif .eg2. The robber stole my friend's bag.Subject: The robberDirect object: the bagPosessive (genitive): my friend'sDER Räuber stahl DIE Tasche MEINES Freundes.