Matthew Inman, who writes a blog under the sobriquet “The Oatmeal,” recently posted a most useful poster called “How to Use an Apostrophe.” You can see it at http://theoatmeal.com/comics/apostrophe, or you can buy an 18”x24” copy to hang on your living room wall for $11.95--but if you don’t want to bother with any of that, here’s a summary:
Don’t use an apostrophe to make a plural, except for letters of the alphabet and numbers. Do use an apostrophe followed by an “s” to indicate a possessive—unless it’s the possessive of a plural already ending in “s,” in which case the apostrophe follows the “s”—or unless it’s the possessive of a pronoun (like his, hers, ours, and its). Do use an apostrophe to indicate contractions (omission of letters).
That’s about it. The big trap to avoid is the its vs. it’s dilemma. It’s is a contraction of “it is” and its is the possessive of it. The reason for this confusion is that when printers started using apostrophes in the sixteenth century, they served three purposes: to indicate the omission of letters, to distinguish a possessive from a plural, and to form a plural of certain words (those ending in vowels and the consonants z, s, ch and sh). For centuries people were unclear about which meaning was intended, and very respectable writers often misused the apostrophe. Washington Irving reportedly used apostrophes to indicate possessives less than half the time, and George Bernard Shaw never used them at all for contractions.
By the way, other grammar posters “The Oatmeal” has available include How to Use a Semi-Colon, When to Use i.e. in a Sentence, and Ten Words You Need to Stop Misspelling.
The Bard of Buffalo Bayou has no posters available, but he will come to your home, often unbidden, and recite his gnomic work, such as the following:
It kindles very little drama
If someone finds a wayward comma;
A colon and a semi-colon
Don’t require the smarts of Solon;
And there’s no problem with the myriad
Misuses of the useful period.
But there’s one punctuation mark
That often leaves us in the dark,
Wondering if it’s expressive
Of contraction or possessive.
And all too often the apostrophe
Results in linguistic catostrophe.