The apostrophe: A tiny mark, not even a punctuation mark, syntactically, but it has its uses. And its misuses! And those misuses can create some true howlers on the page. In the popular
Eats, Shoots & Leaves Lynne Truss talks about the "greengrocer's apostrophe," the wildly inappropriate use of this mark, plus the letter "s," to create plurals on (usually) handwritten signs on fruits, vegetables, flowers, and so on. That use of the apostrohe stems from a misunderstanding of plural and possessive -- but there are other misunderstandings as well.
Let's take a look at this rather underappreciated mark. When a good friend had his work reviewed by a creative writing professor, the professor started off by saying, "There are two words you have a problem with and you must learn the difference between them!" Another friend, an accomplished, award-winning playwright, makes this mistake in every script. What two words? What mistake? It's the mistake about "its"
Apostrophes have two main uses: to take the place of a missing letter in a contraction and to form the possessive form of nouns, and that is the key.
Most of the time, using the apostrophe as a replacement letter is very straightforward. In the "n't" contraction, the apostrophe clearly takes the place of the letter O. Can't, isn't, didn't, doesn't; none of these present much of a problem. Occasionally, people can be confused by constructions such as rhythm n blues and rock n roll, but a quick look in your dictionary of choice will solve that. I prefer rock 'n' roll and rhythm 'n' blues, in which the two apostrophes clearly take the place of the missing "a" and "d" in the conjunction "and," but if another dictionary says otherwise, and you are faithfully following that authority, so be it.
Thus, in "it's," the apostrophe takes the place of the missing "i" in "is." Straightforward, eh?
Not exactly. The pronoun "it" has a possessive form. Just like the proper noun "Oprah" does. And when Oprah has a few miliion dollars, those are Oprah's millions. The apostrophe/s combo indicates possession. And when "it" has a few million dollars, those are . . . its millions. See the trouble? People form the possessive case of nouns with apostrophe/s, and sometimes they form the possessive of the pronoun "it" that way, too.
But pronouns change form, or spelling to indicate possesion rather than adding apostrophe/s: Think of she and her, he and his, they and their, it and its. "It" plus apostrophe/s equals "it is" not "belonging to it."
The easiest way I know to avoid this common mistake is to always think, "Can I write 'it is' instead?" if you can, apostrophe! if not, none. And whatever you do, do NOT rely on spell check. Spell check cannot tell the difference. It's stilll possible for our brain to get some exercise on its own.