The term "N declension" sounds pretty formidable, but the concept is pretty simple: N declensions are primarily masculine nouns that have an "-n" or "-en" added to the end in all grammatical cases except the nominative case.
Included among these masculine nouns that take the N declension are many that describe a person, animal, or nationality, and end in "e."
Schnell infizierte der Hubschrauberpilot auch seinen Kollegen Karsten mit der Lust am Häkeln.
The helicopter pilot quickly infected his colleague Karsten with the desire to crochet.
Captions 28-29, Häkelhelden: Polizisten im Einsatz
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Thus the nominative noun of the person, der Kollege, becomes den Kollegen in the accusative case. How do we know that Kollegen isn't the plural form? If Karsten's colleague Hans had been there too, it would have been "... auch seine Kollegen Karsten und Hans." The ending of the possessive pronoun or the definite article tells you if the noun is plural or not.
Masculine nouns for people that end in "e" and take the N declension also include: der Junge ( "the boy"), der Kunde ("the customer"), der Neffe ("the nephew") and der Hirte ("the shepherd").
Da gibt es zum Beispiel den Drachen der Kontrolle, den Drachen des Ergebnisses und den Drachen der Bedeutung.
For example, there is the dragon of control, the dragon of results, and the dragon of meaning.
Captions 26-28, Deutsch mit Eylin: Mein Bücherregal und Präpositionen
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As you see, the mythical animal der Drache becomes den Drachen in the accusative case. Masculine nouns for animals that end in "e" and take the N declension also include: der Affe ("the ape"), der Bär ("the bear"), der Fink ("the finch"), der Hase ("the hare" or "rabbit"), der Löwe ("the lion"), der Ochse ("the ox"), and der Spatz ("the sparrow.")
Der Sohn eines Russen und einer Radiojournalistin hatte einen Ausreiseantrag gestellt.
The son of a Russian and a German radio journalist applied for an exit visa.
Here, the masculine noun ein Russe becomes eines Russen in the genitive case. Other nationalities that take the N declension include der Grieche ("the Greek"), der Ire ("the Irishman"), der Finne ("the Finn"), der Pole ("the Pole"), der Rumäne ("the Romanian"), der Slowake ("der Slovakian"), der Tscheche ("the Czech"), and der Türke ("the Turk"). Although referring more to an ethnic group or religion than a nationality, der Jude ("the Jew") also takes the N declension.
It may take some time before you start speaking and writing the N declensions correctly. A big part of learning this simply comes from hearing and speaking enough German over the course of several years. And remember: even native German speakers get it wrong occasionally, so don't feel too bad if it seems difficult: it truly is!
Stay tuned for Part II of "The N Declension in German" in the coming weeks.
Further Learning
Yabla published a lesson related to this topic in 2016: Read "Weak Masculine Nouns Not Ending in -E" and see if that helps you to further understand this Yabla German lesson.