Apostrophes indicate possession and form contractions. That's it.

What part of that is unclear? Apparently, the use of apostrophes is extremely confusing to many people. What does "CD's" mean to you? If you answered "It is the plural of CD," you are dead wrong. "CD's" indicates that something belongs to the CD, as in "The CD's case was eaten by a goat." The plural of CD is "CDs." We don't understand when people began using apostrophes to pluralize nouns, but we see signs reading "Driver's wanted" and other such nonsense all the time. We heard, to our dismay, that The New York Times recently used "the 60's" in a headline. Indicating the plural of a decade using an apostrophe (e.g., "70's" instead of "70s" or "seventies") is one of the most common apostrophe errors. We don't care what The New York Times thinks; it's just plain wrong. If you use apostrophes in this manner you must stop doing so immediately.

Here are the right ways to use apostrophes:

Use apostrophes to indicate possession

When you want to indicate possession with a singular noun, whether it ends in an "s" or not, you add an apostrophe and an "s" on the end. For example: "the midget's pathos," "the pathos's source" and "the source's nature." The only exception to this is the word "it," with which you indicate possession by adding an "s" alone (i.e., "its"), because the word "it's" is a contraction meaning "it is." When you want to indicate possession with a plural noun which ends in any letter other than "s," you add an apostrophe and an "s" on the end. For example: "the people's champion," "the hippopotami's excrement," and "the geese's habitat." When you want to indicate possession with a plural noun which ends in "s," you add an apostrophe on the end. For example: "the peoples' champions," "the debutantes' routine," and "the undergarments' impenetrability." That's all there is to say about indicating possession with apostrophes.

Use apostrophes to form contractions

Apostrophes are also used to form contractions. We are told that contractions were invented by sign-painters back in the olden days because they kept running out of room or paint when they were plying their trade. Many commentators suggest that contractions should not ever be used in writing, because they are too informal. We think that contractions are fine in informal writing or when it is necessary to convey a conversational tone, but we'll leave it up to you. The important thing is that you understand that contractions are the only other use for apostrophes. For example: "you can't so don't," "they're wrong," and "it's incorrect to use apostrophes for anything other than indicating possession or forming a contraction."

When can you use an apostrophe to indicate that a noun is plural? Never. We hope that's clear.