Here are the rules:
* Singular nouns: Add an apostrophe and an "s" (e.g., "the dog's bone").
* Plural nouns ending in "s": Add only an apostrophe (e.g., "the dogs' bones").
* Plural nouns not ending in "s": Add an apostrophe and an "s" (e.g., "the children's toys").
* Nouns ending in "s" that are singular: Add an apostrophe and an "s" (e.g., "the boss's desk").
Examples:
* Singular: The cat's tail, the car's engine
* Plural (ending in "s"): The students' books, the birds' nests
* Plural (not ending in "s"): The men's hats, the women's shoes
* Singular (ending in "s"): James's car, the witness's testimony
Exceptions:
* Proper nouns ending in "s": You can either add an apostrophe and an "s" or just an apostrophe (e.g., "Charles's house" or "Charles' house").
Important Note:
Don't confuse possessive nouns with plural nouns. A possessive noun shows ownership, while a plural noun simply indicates multiple things.
For example:
* Possessive: "The cat's tail" (the tail belongs to the cat)
* Plural: "The cats' tails" (there are multiple cats, each with a tail)