Here's a breakdown:
* Singular nouns: We add 's to the end of the noun, regardless of whether it ends in 's' or not.
* Example: the cat's tail, the boss's office, the princess's castle.
* Plural nouns ending in 's': We add just an apostrophe after the 's'.
* Example: the students' work, the dogs' bones, the farmers' market.
* Plural nouns not ending in 's': We add 's to the end.
* Example: the children's toys, the men's room, the women's club.
Exceptions:
* Proper nouns ending in 's': We can add an apostrophe alone or an 's.
* Example: Jesus' teachings, Charles' car, or Charles's car.
* Awkward constructions: When multiple possessives are involved or the possessive is too long, we often use the preposition "of" instead.
* Example: The friend of my sister, instead of *my sister's friend*.
Note: The genitive case is sometimes referred to as the possessive case because it often indicates possession. However, it can also show other relationships, like origin or purpose.