The German Genitive Case

The genitive case is typically the last of the four German cases learned by students and is used when a noun in a sentence belongs to another noun in the sentence. We can usually identify the genitive by thinking “of the” in English. For example: The color of the car is red. In this example “of the car” in German would be in the genitive case. We could also say “The car’s color”, which would also be the genitive. We can not use independent personal pronouns in the genitive. If a noun is in the genitive, we use the following definite, indefinite and negative articles, depending on the gender of the noun:

The genitive case is the only case that can also change the spelling of the noun itself. Masculine and neutral nouns usually add “-s” or “-es”. Feminine and plural nouns do not change. If we need to express possession of the noun that belongs to another noun (the car of my son), then we use the possessive articles in the genitive below. 

Alternative to Genitive

It is important to note that, when indicating possession in German, the genitive case is not always used. In fact, the preposition ‘von’ is often used instead, which is translated as “of” in such cases. For example, to say “my sister’s book” in German, instead of “das Buch meines Bruders,” (genitive)  it could be phrased as “das Buch von meinem Bruder” using the dative preposition. Further, the use of ‘von’ is required when there is no article, for example:

Das Rauchen von Zigaretten ist strengst verboten.
Smoking cigarettes is strictly forbidden.

Nouns that are monosyllabic add ‘es’ to the end (e.g. der Hund → des Hundes); polysyllabic ones just add an ‘s,’ but there is an exception — any masculine or neuter noun that ends in an ‘s,’ ‘ss,’ ‘z,’ or ‘x’ must add an ‘-es’ (e.g. das Gesetz → des Gesetzes).
As far as the monosyllabic nouns that add ‘es,’ there is some variation in spoken German, in which some speakers do not add the ‘e’ before the ‘s.’ Thus, some will say “des Buches,” and others will say “des Buchs.”

Genitive Prepositions
There are some prepositions that will always trigger the genitive case. These can be used as prepositions of time or prepositions of place. 

Examples: 
Sie hat mich innerhalb der Sprechstundenzeit angerufen. (time)
She called me inside of the consultation hours. 

Wir befinden uns innerhalb der Stadtgrenze. (place)
We are inside/within of the city boundaries.