Most nouns that are masculine and end in -e in their nominative singular form are called weak nouns, or schwache Substantive. They are often nouns that refer to nationalities, animals, or professions, such as der Schwede, der Löwe or der Psychologe. Since most German nouns ending in -e are feminine, these exceptions are easy to recognize. What sets them apart from other nouns is the fact that in all cases except for nominative, they end in -en. It is obvious that nouns ending in -e will end in -en in plural cases, but this applies to the singular case too!
Here's examples of the weak noun der Junge with the -en ending in different singular cases, starting with dative:
Der Zauberer sah zu dem zitternden Jungen hinab.
The sorcerer looked down at the trembling boy.
Caption 80, Märchen, Sagenhaft: Der Zauberlehrling
And again in the singular accusative case:
Schiffsverkehr... für einen Jungen aus dem Pott klingt das nur bedingt naheliegend.
Ship traffic... for a boy from the Pit that sounds only conditionally obvious.
Caption 9, Herbert Grönemeyer: stellt sein neues Album vor
And finally in the singular genitive case:
Der Fisch wurde von der Mutter des kleinen Jungen gekauft.
The fish was bought by the mother of the little boy.
Caption 65, Märchen, Sagenhaft: Der standhafte Zinnsoldat
There are also some weak nouns that end in letters other than -e, but more about them later!
Go to this page to practice your weak noun endings, and visit Yabla German to find more examples of weak masculine nouns in practice.