What are the rules for forming possessive nouns with plural nouns?

The rules for forming possessive nouns with plural nouns are as follows:

1. If the plural noun does not end in "s," add an apostrophe followed by "s" to indicate possession. For example, "The children's toys were scattered all over the room."

2. If the plural noun already ends in "s," simply add an apostrophe after the "s" to indicate possession. For example, "The students' backpacks were left in the classroom."

3. If the plural noun is an irregular plural that does not end in "s," add an apostrophe followed by "s" to indicate possession. For example, "The men's coats were hanging in the closet."

4. If the plural noun is a compound noun, add an apostrophe and "s" to the last word of the compound to indicate possession. For example, "The mothers-in-law's opinions differed on the matter."

5. If the plural noun is a proper noun, follow the same rules as above. For example, "The Smiths' house was beautifully decorated for the holidays."

It is important to note that when indicating joint possession, where two or more people share ownership, the apostrophe and "s" are added only to the last noun. For example, "John and Mary's car is parked in the driveway."