Some masculine nouns and one neuter noun (das Herz) add an ‘(e)n’ ending in the accusative, dative, and genitive cases. There are several categories that these nouns fall under, including: nationalities formed from adjectives (e.g. der Franzose), ages (e.g. der Junge), animals ending in ‘e’ (e.g. der Löwe), professions ending with an ‘e’ (e.g. der Biologe), nouns ending with ‘ant,’ ‘ent,’ or ‘ist’ (e.g. der Praktikant, der Präsident, der Spezialist), and several others not grouped by any specific category.
Here are some examples of weak masculine/neuter nouns and their declensions through all four cases:
Note that “das Herz,” the weak neuter noun, is somewhat irregular in that the ‘en’ ending does not apply to the accusative case AND the genitive ‘s’ is added (the latter only occurs in a handful of nouns, including “der Wille,” “der Name,” “der Glaube,” and “der Gedanke”).