Rule 1
Use the apostrophe with contractions. The apostrophe is always placed at the spot where the letter(s) has been removed.
Examples:
don't, isn't
You're right.
She's a great teacher.
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 actress's hat
one child's hat
Ms. Chang's house
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.
Examples:
Mr. Jones's golf clubs
Texas's weather
Ms. Straus's daughter
Jose Sanchez's artwork
Dr. Hastings's appointment (name is Hastings)
Mrs. Lees's books (name is Lees)
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 immediately use the apostrophe.
Examples:
two boys' hats two women's hats
two actresses' hats
two children's hats
the Changs' house
the Joneses' golf clubs
the Strauses' daughter
the Sanchezes' artwork
the Hastingses' appointment
the Leeses' books
Rule 5
Do not use an apostrophe for the plural of a name.
Examples:
We visited the Sanchezes in Los Angeles.
The Changs have two cats and a dog.
Rule 6
With a 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.
Examples:
Cesar and Maribel's home is constructed of redwood.
Cesar's and Maribel's job contracts will be renewed next year.
Indicates separate ownership.
Cesar and Maribel's job contracts will be renewed next year.
Indicates joint ownership of more than one contract.
Rule 9
Never use an apostrophe with possessive pronouns: his, hers, its, theirs, ours, yours, whose. They already show possession so they do not require an apostrophe.
Correct:
This book is hers, not yours.
Incorrect:
Sincerely your's.
Rule 10
The only time an apostrophe is used for it's is when it is a contraction for it is or it has.
Examples:
It's a nice day.
It's your right to refuse the invitation.
It's been great getting to know you.
Rule 11
The plurals for capital letters and numbers used as nouns are not formed with apostrophes.
Examples:
She consulted with three M.D.s.
BUT
She went to three M.D.s' offices.
The apostrophe is needed here to show plural possessive.
She learned her ABCs.
the 1990s not the 1990's
the '90s or the mid-'70s not the '90's or the mid-'70's
She learned her times tables for 6s and 7s.
Exception:
Use apostrophes with capital letters and numbers when the meaning would be unclear otherwise.
Examples:
Please dot your i's.
You don't mean is.
Ted couldn't distinguish between his 6's and 0's.
You need to use the apostrophe to indicate the plural of zero or it will look like the word Os. To be consistent within a sentence, you would also use the apostrophe to indicate the plural of 6's.
Rule 12
Use the possessive case in front of a gerund (-ing word).
Examples:
Alex's skating was a joy to behold.
This does not stop Joan's inspecting of our facilities next Thursday.
Rule 13
If the gerund has a pronoun in front of it, use the possessive form of that pronoun.
Examples:
I appreciate your inviting me to dinner. I appreciated his working with me to resolve the conflict.
Those three are generally correct, but there are fiddly bits.
Don't use one for possession when using pronouns. Hers, His, Its, Yours. There are probably more.
It's means It Is. Its means the It in question possesses something.
There's some discussion on this one, but as a rule, when using one for possession, and there's an S at the end of the word, it goes after. My grandparents' place.
The debate, for the record, is whether doubling up is acceptable. IE: grandparents's. Generally not accepted, but it is a discussion.
It's accepted practice, and I highly recommend it, to begin a word that has been shortened on the starting end with an apostrophe. For example: Because becomes 'Cause or Until becomes 'til.
Incidentally, using till when people mean 'til is a pet peeve of mine. Till is the word meaning to turn earth for planting. 'Til is the short form of until.