In many languages, a noun's form stays fixed regardless of its role in the sentence — case is carried by word order or by a preceding article, while the noun's body never changes. German case marking usually works similarly: Nominativ, Akkusativ, Dativ and Genitiv are signaled mainly by the article (der, den, dem, des) while the noun itself stays constant. A special class of masculine nouns breaks this pattern: they don't stop at the article — the noun itself also takes an -n or -en ending in every case except the Nominativ singular. This group is known as the n-Deklination, or "weak nouns" (schwache Substantive), and it trips up even advanced learners precisely because it is unpredictable relative to every other masculine noun.
Where do these nouns come from? Most belong to recognizable categories: nouns for people and professions frequently ending in -e (der Kollege, der Junge, der Nachbar), animals (der Löwe, der Bär, der Affe), nationalities (der Türke, der Franzose), and words of Latin or Greek origin ending in suffixes such as -ist, -ent, -ant or -at (der Student, der Präsident, der Journalist, der Demokrat). Recognizing these morphological patterns lets you predict which new noun will behave this way without memorizing each one individually.
The operating rule, once understood, is simple: the Nominativ singular carries no extra ending (der Student), but the Akkusativ, Dativ and Genitiv singular all take -en (den/dem/des Studenten). In the plural, these nouns generally behave like any other -en plural, so no distinction is felt there. Two irregulars deserve special attention: der Herr takes only -n in the singular (den Herrn) but -en in the plural (die Herren); and der Name joins this group but additionally adds -s in the Genitiv singular (des Namens) rather than -n alone — an exception it shares with related words like der Gedanke and der Glaube.
The most common mistake among advanced learners is inflecting only the article and leaving the noun itself untouched, producing the incorrect Ich sehe den Student or Ich helfe dem Nachbar instead of the correct Ich sehe den Studenten and Ich helfe dem Nachbarn. Forgetting the -n/-en ending on the noun itself is typically the first thing an examiner's eye catches, because it signals a failure to grasp that "weakness" in these nouns belongs to the noun, not merely to the article that precedes it.