They don't really function like any of the above, however. There are two primary uses for apostrophes:
- To make a contraction
- To make a noun possessive
Seems simple, no? It is, for the most part. Let's take a look:
The book of Steve sits on the coffee table.
Sounds like a book of the Bible, I imagine. But unfortunately, Steve is no prophet. He also didn't write the book, but instead just owns it. This "book of Steve" just sits on his coffee table in an open spot so all can see how smart Steve is and see him for the intellectual that he wants to be--I mean, is. But enough about Steve. Let's talk about his book.
Not only does saying "book of Steve" make it sound highly revered, it's also wordy as heck. Let's make it less wordy:
Steve's book sits on the coffee table.
Wow. Cuts down on the words, doesn't it? Also makes it a bit clear as to what the nature of the book is. This isn't the divine word of Steve, but just a book he owns, a very thick book that makes him look like he's a smarty. But what about the contractions?
I do not believe in the word of Steve.
Sounds very formal, doesn't it? But maybe we're talking with our best bro Jake who doesn't much care for high-and-mighty language. Let's shave it down for him.
I don't believe in the word of Steve.
Makes it only slightly less wordy, but definitely feels more conversational and less stiff. Then again, Jake might word it like this:
I ain't a believer in the word of Steve.
But there's a trap with all this business: it. What do we do with it? The contraction of it and is is simple enough: it's. But we can't do that with the possessive form. As far as I can tell, the reason for making the possessive form of it lack an apostrophe is just to minimize the confusion. I can see where it might occur.
Its unlikely that Steve will ever see this post. Its web address is rarely typed on his computer.
It is one of those rare words that can so often be used in both a contraction and as a possessive pronoun. You wouldn't see that confusion with the pronoun "he" since its possessive form is "his"--an entirely different word. The omission of the apostrophe in the possessive form of it is just a nice, although understandably confusing, way of differentiating between the two words.
I can see why some have problems with apostrophes. It's an aspect of grammar that you learn so long ago that it just feels increasingly irrelevant the older you get. On the whole, though, I'd say most people don't really have a problem with them. I think many issues with apostrophes may come from simple proofreading mistakes, but I can only posit this as a theory so far.
Then again, this site may suggest otherwise.