An easy way to determine if you need a possessive form of a noun is to create an “of the” phrase. For example, the girl’s hat becomes the hat of the girl. If the noun following “of” is an object or building, you do not need to create a possessive. If the noun is anything other than an object or building, you will need to use the following rules for creating a possessive.
At first glance, creating a possessive may seem like a simple matter of adding ‘s.
• The dog’s bark was loud.
• The boy’s hat was red.
Even many singular nouns ending in -s can be made possessive by adding ‘s.
• Fluffy is James’s cat.
• Tom is Mrs. Jones’s attorney.
Of course, there are exceptions to every English grammar rule. In some cases, pronouncing the possessive ending can be somewhat awkward when the noun ends in -s. In these instances, it is acceptable to simply add the apostrophe. It is up to the writer to make the judgment call and to punctuate according to pronunciation.
Many writers also make exceptions for the possessive of ancient proper names ending in -es, -is, and -us.
For plural nouns ending in -s, you simply add the apostrophe.
• two countries’ laws
• three dogs’ toys