Masculine and neuter substantives get the suffix -s in Genitive Singular.
Sometimes it can be -es instead of -s. How can we know when to use -s and when -es?
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The following substantives get the suffix -es:
- Substantive that end in -s, -ß, -x, -tsch, -tz, -z, -zt (e.g. das Glas – des Glases)
- many monosyllabic substantives (e.g. das Volk – des Volkes)
- Substantive that end in -nis and -us (e.g. der Bonus – des Bonusses)
Substantive that end in -us and -os do not get any suffix in Genitive Singular (e.g. das Epos – des Epos).
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