What is the genitive case?

In most cases, the genitive case is used to denote possession e.g. this is my dad's dog. The article of the 'possessor' changes according to the gender of the noun, but the article of the 'possessed' noun doesn't change! The genitive case can be used in the following way;Ich habe das Buch des / eines Mannes (I have the / a man's book)Ich habe das Buch der / einer Frau (I have the / a woman's book)Ich habe das Buch des / eines Mädchens (I have the / a girl's book)In this case, that which is possessed (Das Buch) is in the accusative case, whilst the possessor is in the genitive case. In the genitive case, the article always changes and - if the noun is masculine or neuter - the noun ending changes by adding an (s) or an (es). See the rules below;ARTICLESDer --> Des / Ein --> EinesDie --> Der / Eine --> EinerDas --> Des / Ein --> EinesNOUN ENDINGSDer Mann --> des Mannes (add 'es' after 1 syllable words or those already ending in '-s')Die Frau --> der Frau (no change)Das Mädchen --> Des Mädchens (add 's' after 2+ syllables words or those already ending in '-e')