Having already learned the prepositions that require the accusative or the dative, B2 introduces a further group that requires an entirely different case: the genitive, which marks possession or an indirect relationship of belonging rather than location or movement. Its effect is most visible in how the definite article and the ending of masculine and neuter nouns change.
The key prepositions here are trotz (despite), which signals that a result is unaffected by the stated cause; während (during), which marks a simultaneous time frame; wegen and aufgrund, both expressing cause, though aufgrund is more formal and typical of reports and administrative writing; (an)statt, which expresses substitution or a preferred alternative; and innerhalb / außerhalb, which mark spatial or temporal boundaries, inside and outside.
Morphologically, the central rule is this: with masculine and neuter nouns, the article der or das becomes des, and the noun itself takes an -s or -es ending (especially with one-syllable nouns), as in des Mannes or des Kindes. With feminine nouns and plurals, only the article changes, to der, while the noun itself stays unchanged, as in der Frau or der Kinder. This extra change to the noun's ending is what fundamentally sets the genitive apart from the dative and accusative, which usually only change the article.
It is worth knowing that the genitive belongs more to written and formal registers than to everyday spoken language; in casual speech, German speakers frequently replace the genitive with the dative after wegen or trotz, saying wegen dem Regen instead of wegen des Regens. This usage is considered non-standard and is marked wrong in exams and formal or academic writing, which is exactly the register this lesson targets.
The most common mistake, especially for learners whose native language has no comparable case system that reshapes both noun and article together, is forgetting to add -s or -es to a masculine or neuter noun after des, producing the incorrect des Mann instead of des Mannes. Remember that the genitive always demands two simultaneous changes — article and noun — with masculine and neuter nouns.