Apostrophes remind me of comma bats, commas that hang upside down. There are rules that decide where and when apostrophes are used.
Rule 1. Use an apostrophe in 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 an apostrophe to show possession. Place the apostrophe before the s to show singular possession, unless the singular noun ends in an s, then place the apostrophe after the final s.
Examples: one girl's book; one woman's hat; one actress' hat; one child's hat; Ms. Chang's house
NOTE: Nouns ending in s or an s sound are not required to have the second s added in possessive form.
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 girls' books; two women's hats; two actresses' hats; two children's hats; the Changs' house; the Joneses' golf clubs; the Strauses' daughter; the Rogeres' car.
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 her 6's and 0's.(You don't mean Os.)
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.
Knowing where and when to use those bat commas is a matter of study and practice.