First, contractions. This sounds obvious, and it’s easy to remember where the apostrophe goes in words like don’t and wasn’t. But sometimes, it gets confusing. Anytime you are leaving a letter out of a word, put the apostrophe where you left out that letter. As an example from above, I wrote “exciting’s probably not the right word,” combining exciting and is, and putting an apostrophe in place of the I. If you aren’t sure where you’re supposed to be putting your apostrophe, think about the two words separately, and then you’ll know. Like the commonly butchered word y’all, for instance. Y’all is you+all combined. You are removing the ‘ou’, so the apostrophe needs to go after the Y. Not after the A*. Apostrophe using omissions also include years, like: "class of ’11", which leaves out the 20 in 2011.
Next, forming possessives. Again, this sounds simple, but it can be confusing. Basically what helps me remember whether or not an apostrophe belongs is to rearrange the sentence in my head. For example, if I write the sentence, “I am going to Mary’s house.” I could rewrite it as “I am going to the house belonging to Mary.” If you can’t rearrange the sentence so that the word with the ‘s possesses something, you don’t need it there. And don’t forget to think about whether the apostrophe goes before or after the S. For example: “I am going to the smith’s house” vs. “I am going to the Smiths’ house.” In the first sentence, I would assume you’re going to see a blacksmith (or other type of smith.) And in the second, I would assume you’re going to the home of a family with the last name Smith.
It’s also important to think about when not to use apostrophes. Just because a word ends with an S doesn’t necessarily mean you need an apostrophe there. No apostrophe is required in a sentence like, “I saw the Smiths when I was out for a walk.” They are plural, yes, but they aren’t possessing anything in this sentence. And when you are referring to two people possessing the same item – for example, “I went to Mary and Matt’s house.” – you don’t need an apostrophe after the first name. And do not use apostrophes with possessive pronouns (his, yours, its, etc.) because they already show possession as they are.
Apostrophes (not apostrophe’s!) come up so often when we write that they seem really basic and get forgotten about, but it’s good to refresh ourselves every now and again, isn’t it?
And for a slightly more thorough version of what I said, in flowchart form (with pictures!), visit The Oatmeal.
*Really you guys. I think I see this done wrong more often than I see it done right.