Why does the noun "Der Junge" become "Den Jungen" in the accusative case whereas "Der Mann" just becomes "Den Mann"?

"Der Junge" is an example of something called a Weak Masculine Noun. This sounds more complicated than it actually is. Basically, in German, most nouns are Strong, which means they decline (change their ending) in a regular pattern. However, there are a small group of Masculine Nouns which are Weak. They typically end in -e, such as "Der Neffe" (Nephew) or "Der Name" (Name) and all that happens with them is that they add an -n in every case EXCEPT for the Masculine Singular. Take a look at the example below:Nominative Sing. - Der Junge Nominative Plu. - Die JungenAccusative Sing. - Den Jungen Accusative Plu. - Die JungenDative Sing. - Dem Jungen Dative Plu. - Den JungenGenitive Sing. - Des Jungen Genitive Plu. - Der JungenAnd that's all there is to it! If you follow this pattern, you won't be confused by these pesky weak masculine nouns every again!