When it comes to writing, this is one that really pops out when you make a mistake. You add an apostrophe when you really meant plural, or you leave it out by mistake. It’s not as tough to get right as you might think; in fact, it’s downright easy.

Apostrophes are used to show ownership, not plural. (Yes, I hear some of you arguing already. I’ll get to that.)

So, if you’re talking about more than one, add an s. If you’re talking about ownership, you’ll need an apostrophe. Let’s look at some examples.

1. John owns four cars. (simple, right?)
2. None of John’s cars actually runs. (Ownership, the cars belong to John.)
3. The children’s playground is a mess. (The playground belongs to the children.)

At it’s most simple, showing possession is done by adding ‘s. If a word doesn’t already end in s, make it possessive by adding ‘s, whether it’s singular or plural. Here are some examples.

1. The cheerleader’s performance was almost perfect. (One cheerleader).
2. The book’s binding needs repair. (One book.)
3. The men’s basketball team has had its best year ever. (Men is plural, but it doesn’t end in s.)

But what if a word ends in s but isn’t plural? How do you make it possessive? You go ahead and add the ‘s anyway. You can hear this one. You say “boss-es temper,” and “Charles-es wife” below, right? Let that help you remember to add the ‘s.

1. His boss’s temper was legendary.
2. Charles’s wife insisted he take the promotion.

If a word is already plural and ends in s, then just add the apostrophe.

1. The cheerleaders’ performance was almost perfect. (The performance was by more than one cheerleader).
2. The sled dogs’ feet were in need of medical care after the race.
3. My brothers’ cars were totaled within days of each other. (More than one brother wrecked more than one car.)

Let’s review some times when you shouldn’t use the apostrophe.

1. Don’t use the apostrophe when all you mean is plural.
2. Don’t use the apostrophe with pronouns (theirs, yours, his, its, etc.)
3. Don’t confuse its and it’s.

Here’s the exception I alluded to earlier. There is one case when you do use the apostrophe for plurals. Use an apostrophe to show the plural of a lower case letter.

1. She’s learning her a, b, c’s.
2. You didn’t dot any of your i’s in this essay.

There’s an interesting discussion here about apostrophes used to show plurals, which goes to show that even the “experts” don’t agree on many of the finer points of grammar. In my opinion, err on the side of clarity. I’d write the following, even though some might argue that I’m breaking the “rules.”

1. You got three F’s on your report card. (F is upper case, but I think it’s easier to read with the apostrophe.)
2. You use too many and’s in your writing. (I think this is easier to read than ands.)

You can find justification for either way of doing it. Just pick one and be consistent.

Finally, what do you do with hyphenated words?

1. The mothers-in-law have a meeting night at church. (More than one mother-in-law, but no possession.)
2. My mother-in-law’s car is in the garage. (One mother-in-law, showing possession.)
3. The mothers-in-law’s meeting lasts about two hours. (Plural and possessive. While this is technically correct, I find it awkward. I’d rewrite to avoid it entirely.)

Summary

1. Add s to show plural
2. If a word does not already end in s, then add ‘s to show possession. This applies whether the word is singular or plural.
3. If a word ends in s already and is plural, add only the apostrophe.
4. If a word ends in s already and is singular, add ‘s to make it possessive.