If you're confused over the proper use of apostrophes, you're not alone.
In a recent questionnaire, I asked my newsletter readers if there were some topics they'd like me to cover. One reader said "Grammar! And please start with the use of apostrophes."
Since the explanation is a bit long for the newsletter, I added a simple answer with a link to this post…
How to use apostrophes
Apostrophes have two functions. One is to stand in for letters that are missing. In the most common use, they form a contraction - such as turning can and not into can't or will and not into won't.
Contractions are useful because they slightly alter the meaning or "feel" of a statement. They're a bit more casual than the words written separately.
Think back to being a kid. If you asked for a second bottle of soda pop (or a pony, or violent video game, or ??) and Mom said "No you can't have one," you might think that if you ask again later she just might give in. If she said "No, you can not," It sounded a bit more like the final answer.
In the context of standing in for missing letters, apostrophes are also used to write slang, or a local dialect. Here's an example from Grammar Girl: "I saw 'em talkin' yonder," with apostrophes to indicate that the speaker said 'em instead of them (t-h-e-m), and talkin' instead of talking (t-a-l-k-i-n-g).
The other is to create a possessive, and while we sometimes see an apostrophe stuck into the middle of a word where it doesn't remotely belong, (as in ladie's for ladies') where we see it misused most often is when writers use an apostrophe to form a plural.
Plurals are formed by adding an "s" (dogs, cars, boats) or "es" (dresses, businesses, glasses).
Possessive's are formed by adding 's (apostrophe s) or simply an apostrophe.
(As you see below, the plural of some words (lady) is formed by dropping a y and adding ies, but that's a whole other topic. Our focus here is on the fact that plurals are NOT formed with apostrophes.)
More than one agent: agents
Belonging to an agent: agent's
Belonging to a group of agents: agents'
More than one cat: cats
Belonging to a cat: cat's
Belonging to more than one cat: cats'
More than one lady: ladies
Belonging to one lady: lady's
Belonging to more than one lady: ladies'
Notice that if the noun already ends in s – as in ladies – the apostrophe just gets tacked on to the end. You don't need to add another s.
Exceptions always cause complications:
The only exceptions to this rule about possession being formed by an apostrophe are hers and yours and theirs and its. The first three don't usually give much trouble, but many confuse its and it's. There is no apostrophe when you mean "belonging to it." You simply write "its." Try to link it up in your mind with hers and yours and theirs.
"It's" is still the contraction – the joining of the words "it" and "is."
One rule that you might keep in mind …
If you see an apostrophe s, or an apostrophe at the end of a noun ending in s, something has to follow that could belong to the noun.
The car's bumper.
The cat's scratching post.
The agent's reputation.
A statement such as "There were 6 car's in the lot" is obviously incorrect. Six car's what? It could perhaps be six car's tracks in the snow, but it has to be something that belonged to those six cars.
If it's firm in your mind that an apostrophe s means "belonging to," you'll start to see signs that make you laugh. These are especially common in grocery stores, small cafes, and neighborhood shops. They read something like:
You'll ask yourself "Green grape's what is $2.49?"
I once saw a commercially printed 4' X 8' sign on the exterior of a building that read "Get your box's here." I did laugh out loud at that one.
For more on this topic, visit Grammar Girl at: http://www.quickanddirtytips.com/education/grammar/apostrophe-catastrophe-part-one
Images courtesy of Stuart Miles @ freedigitalphotos.net