German adjectives change their endings depending on three factors: the case (Nominativ, Akkusativ, Dativ, Genitiv), the gender and number of the noun, and the type of article that precedes them. This produces three systems of declension. In the weak declension (after the definite article der, die, das), the article already carries the case information, so the adjective only adds -e in the nominative singular and -en in most other positions: der alte Mann, die junge Frau, das kleine Kind, die netten Leute, den alten Mann, dem alten Mann.
The mixed declension occurs after the indefinite article ein, eine, ein. Here, the article has no ending in the masculine and neuter, so the adjective takes over: ein alter Mann (-er), eine junge Frau (-e), ein kleines Kind (-es). In the accusative, the masculine becomes einen alten Mann (-en), feminine stays eine junge Frau (-e), and neuter remains ein kleines Kind (-es). When there is no article at all, the strong declension kicks in, and the adjective endings match those of the definite article itself: kalter Kaffee (-er), frische Milch (-e), warmes Bier (-es), alte Freunde (-e). In the dative after definite articles, every adjective ends in -en: dem alten Mann, der jungen Frau, dem kleinen Kind, den netten Leuten.
Separately, a small group of masculine nouns follows the N-Deklination: they add -(e)n in every case except the nominative singular. Common examples are der Junge (declined der Junge, den Jungen, dem Jungen, des Jungen), der Herr (declined with just -n: der Herr, den Herrn, dem Herrn, des Herrn), and der Kollege, der Student, der Kunde, der Nachbar. Because they mostly refer to people (often with professional or social roles), they tend to be the nouns that appear most often in B1 conversations about work and society. Recognizing these patterns is essential for producing grammatically correct noun phrases and avoiding the most common adjective-article agreement errors.