Des Genitivs
The least common shiny grammatical case is the genitive case, just like the dative case it comes into use in sentences which describe posessions or belongings, however in the case of the genitive case, it's only used on combinations of two direct objects or beings, for instance to underline the posession of some almonds:
The genitive articles for masculine or neuter nouns are des and eines. Feminine nouns in genitiv are identical to dative, receiving der and einer. Unlike dative, but similar to nominative, plurals follow the feminine and take der as well.
Notice how das Eichhörnchen changes into des Eichhörnchens? This is because for polysyllabic neuter and masculine nouns end in -s in the genitive case.
Masculine | Feminine | Neuter | |
Nominative | der Mond | die Schlange | das Sauerkraut |
Genitive | des Mondes | der Schlange | des Sauerkrauts |
Of course there are also some exceptions in place with the polysyllabic and monosyllabic nouns mentioned before. For instance des Sauerkraut is a polysyllabic neuter noun in its genitive form, would we take the monosyllabic variant of just "Kraut", the noun would follow with the -es ending: des Krautes.
Keep in mind that the -es ending would not apply to words which already end in a vowel like in the case with der Uhu, a monosyllabic masculine noun. The correct genitive would be des Uhus in this case.
Some nouns already end in -s so one might be inclined to add an apostrophe there, the German language however doesn't use apostrophes so the noun wouldn't change in this case.