Rule 1
Use the Apostrophe with contractions. The Apostrophe is always placed at the spot where the letter(s) have been removed. Examples : don't, isn't, you're are right.

Rule 2
Use the Apostrophe to show possession. Place the Apostrophe before the s to show singular possession. Examples : one boy's hat, one woman's hat, one child's hat.
Note : Although names ending in s or an s sound are not required to have the second s added in possessive form, it is preferred. Texas' weather, Straus's daughter.

Rule 3
Use the apostrophe where the noun that should follow is implied. Example : This was his father's, not his jacket

Rule 4
To show plural possession, make the noun plural first. Then use the apostrophe immediately. Examples : two boys' hats.

Rule 5
Do not use the apostrophe for the plural of a name. Example : The Mishras have two cats and a dog.

Rule 6
With singular compound noun, show possession with s at the end of the word. Example : my mother-in-law's hat,

Rule 7
If the compound noun is plural, form the plural first and then use the apostrophe. Example : my two brothers-in-law's hats

Rule 8
Use the apostrophe and s after the second name only, if two people possess the same item. Example ; Ram and Krishna's home is far away. Ram and Krishna's contracts will be renewed next year. indicates separate ownership.

Rule 9
Never use an apostrophe with possessive pronouns : hers, yours, his, its, theirs, ours, whose. They already show possession so they do not an apostrophe. Example : This book is hers, not yours.

Rule 10
The only time an apostrophe is used for it's is when it is for a contraction for "it is" or "it has". Example ; It's a nice day. It's your right to refuse. It's has been great to know you.

Rule 11
The plurals for capital letters and numbers used as nouns are not formed with apostrophes. Example : She consulted with three MDs but she went to three M.D's offices. The apostrophe is needed here to show plural possessive. She learnt her ABC's, 1990s are not the 1990's. She learned her time tables for 6s and 7s.
Exception : use apostrophe with capital letters and numbers when the meaning would be unclear otherwise. Example : Pleas dot your i's, Sita could not distinguish between her 6's and 0's.

Rule 12
Use the possessive case in front of gerund (-ing word). Example : Rita's skating was a joy to behold. This does not stop principal's inspecting of our college----

Rule 13
If the gerund has a pronoun in front of it, use the possessive form of that pronoun. Example : I appreciate your inviting me to dinner, I appreciated his working with me