What are rules forming the possessive of singular noun and plural noun?

Singular nouns form the possessive by adding an apostrophe s ('s) to the end of a noun; for example:

"My mother's cakes are the best!"

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word; for example:

"Put it on the boss' desk."

Add an apostrophe s ('s) after the existing s at the end of the Word; for example:

"Put it on the boss's desk."

The -s's is the most commonly used, but if you are a student, use the form that is preferred by your teacher.

Plural nouns that end in -s form the possessive by adding an apostrophe (') after the existing -s; for example:

"The teachers' meeting is at four."

For irregular plural nouns that don't end with -s, the possessive is formed by adding the apostrophe s ('s) the same as a singular noun; for example:

"The children's playground is freshly painted."