Recognize an apostrophe when you see one.
Although the apostrophe might look like a comma defying gravity, this mark of punctuation has three distinct jobs: to show possession, make contractions, and form odd plurals.
Use the apostrophe to make words possessive.
The donut belonging to Vince
The hot sports car owned by Sylvia
The home of the Watsons
Showing possession with an apostrophe [and often an added s] simplifies the phrasing:
Sylvia's hot sports car
The Watsons' home
To use the apostrophe to show possession, follow these guidelines:
Nouns Without a Final S
When a noun does not end in s, use apostrophe + s.
The lollipop belonging to Elena = Elena's lolllipop
The dictionary owned by the grammar whiz = The grammar whiz's dictionary
The shoes of the children = The children's shoes
Nouns with a Final S
Nouns that do end in s require more thought. If the noun is plural, just attach an apostrophe.
The leashes for the dogs = The dogs' leashes
The grades of the students = The students' grades
The wages earned by the workers = The workers' wages
The location of the apostrophe clues an educated reader about numbers. Take the noun dog, for example. If the apostrophe comes before the s, we know that one dog has multiple leashes:
The dog's leashes
But if the apostrophe comes after the s, we understand that many dogs have a variety of leashes:
The dogs' leashes
If the noun ending in s is singular, most sources recommend adding both the apostrophe and an additional s.
The bad luck that plagues Odysseus = Odysseus's bad luck
The strong tentacles of the octopus = The octopus's strong tentacles
The directive given by the boss = The boss's directive
In these cases, the apostrophe + additional s adds another syllable to the pronunciation. If the extra syllable sounds unnatural, add just the apostrophe.
So you can write ...
Odysseus's bad luck
because Odysseus-ES sounds okay, but you just need...
Achilles' hot temper
because Achilles-EEZ sounds weird.
Since "sounds natural" can be a matter of region or opinion, your best bet in these cases is to consult the textbook or handbook assigned to your class, the teacher or professor who will be evaluating your assignment, or your supervisor or the style manual your industry uses. Then follow the advice that you get.
Use the apostrophe to make contractions.
The apostrophe indicates when a writer has combined two or more words into a single word. Here are some examples:
Cannot = Can't
Does not = Doesn't
Let us = Let's
I am = I'm
It is = It's
We have = We've
They are = They're
He will = He'll
She has = She's
You had = You'd
They would have = They'd've
If you are writing for a very formal audience—a teacher, a boss, an editor, a snotty group of intellectuals—you might want to err on the side of stuffy and spell out the words rather than contract them:
We'd've We would have arrived on time if our car hadn't had not gotten a flat tire.
Use the apostrophe to make odd plurals.
Whenever you have plural lower-case letters, use the apostrophe + s to make the letters plural.
Grandma prefers to sign birthday cards with k's and h's instead of x's and o's.
Do you remember how many t's are in the word commitment?
If you have capital letters, however, most writers use just the s.
David has two BAs—one in art history and the other in classical studies—but no job.
Sharon has two DOBs, the official one on her birth certificate and the one four months later when her great aunt Matilda remembers to send a check.
Some writers will use apostrophe + s to make capital letters plural to avoid confusion.
Look at all of those I's in your project summary. You did have two other teammates, right?
Kevin earned three A's this semester, missing a 4.0 because of one missed question on Dr. Grayson's final exam.
To make numbers plural, most writers use only an s.
To escape the high 90s in town, we drove to the beach to enjoy the cool ocean breeze.
If you give me all 20s, my pocket will bulge with cash!
You will, however, see some writers use apostrophe + s.
To escape the high 90's in town, we drove to the beach to enjoy the cool ocean breeze.
If you give me all 20's, my pocket will bulge with cash!
What's best—90s or 90's, 20s or 20's? Whatever your textbook, teacher, boss, or industry style guide recommends. Decide who is your intended audience, and then use what that person or group expects.
Other Parts of Speech Used as Nouns
Sometimes you will need to use another part of speech as a noun. If the expression is common, use just an s to make it plural.
The haves opened their lunch bags and began munching in front of us have-nots.
My favorite reality shows focus on befores and afters.
But if you use another part of speech as a noun in a less familiar way, you can form the plural with apostrophe + s.
With her red pen, Dr. Pennington crossed through all of the well's I had used as transitions.
A chorus of ah-ha's filled the classroom as Prof. Warner finally solved the difficult equation on the board.
Remember that just because a word ends in s doesn't mean it must have an apostrophe.
Some people get so used to seeing apostrophes with s's that they think every word that ends in s requires an apostrophe. Don't make this mistake!
For example, singular present tense verbs end in s but do not need any punctuation.
Sheila know's knows that Daniel does not have enough color sense to buy the house paint unsupervised.
Most plural nouns end in s, but unless they are possessive, you don't include the apostrophe.
The monkey's monkeys wanted the students' juice boxes.
Do not touch that cupcake. It is her's hers.
Can we borrow some pencils? We forgot our's ours.
These are Frank's camping supplies. Their's Theirs are still in the trunk.