der Junge
boy
A male child or a young man in adolescence.
Der Junge spielt mit seinem Ball im Park.
The boy is playing with his ball in the park.

boy
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boy
A male child or a young man in adolescence.
Der Junge spielt mit seinem Ball im Park.
The boy is playing with his ball in the park.

'Der Junge' (the boy) is a weak noun (N-declension). This means it gets the ending '-n' or '-en' in all cases except the nominative singular. This is a very common mistake.
Ich helfe dem Jungen bei den Hausaufgaben.
I help the boy with his homework.
The noun 'der Junge' (the boy) is derived directly from the adjective 'jung' (young). It literally describes a young male person.
Mein kleiner Bruder ist noch sehr jung.
My little brother is still very young.
As an exclamation or form of address, 'Junge!' (boy!) or 'Junge, Junge!' (boy oh boy!) is used colloquially to express surprise, a warning, or sometimes annoyance.
Junge, Junge, war das ein spannendes Spiel!
Boy oh boy, that was an exciting game!
If you play cards in Germany, especially Skat, 'der Junge' can mean the playing card 'Bube' (English: jack). The context of the game makes the meaning clear.
Mit den beiden Jungen auf der Hand konnte er das Spiel gewinnen.
With the two jacks in his hand, he was able to win the game.
man
'der Mann' and 'der Junge' both refer to male people. The important difference is age: a man is an adult, while a boy is a child or teenager. The words can be confused because both are male, and 'Junge' is sometimes also used for a very young man. In neutral situations, it is better to say 'Mann' for an adult person. For a child or teenager, 'Junge' is more natural.

Im Bus liest der Junge ein Buch für die Schule.
On the bus, the boy is reading a book for school.
Der Mann hilft seiner Nachbarin, die schweren Taschen zu tragen.
The man helps his neighbor carry the heavy bags.