A friend posted a pet peeve on her Facebook the other day: using it's and its incorrectly.
I agree. It bugs me to no small extent, especially when, unlike many English grammar rules, this one is easy enough to remember and use. So for the sake of other authors (as I've seen it misused by authors too often) as well as for anyone who cares about her grammar being correct and non-irritating, here's the rule:
When it's is a contraction, meaning it is, use the apostrophe: It's raining today.
When its shows possession, on the other hand, leave the apostrophe out: The rain made its leaves shiny.
Simple. Of course, it doesn't work that way with other contractions. For instance, "John's going to work" and "John's boat is orange" both use the apostrophe. So the only thing to remember is that if ITS shows possession, DON'T use an apostrophe. If you can change it to "it is" use it, but not otherwise.
Other apostrophes that mess people up are with plural possessives. A lady's dress is straightforward, but what about when there's more than one lady? If we have 3 ladies with 3 dresses, it would be: The ladies' dresses. However, if the one lady's name is Jess, it would be Jess's dress, just like it would be Joan's dress. Simple?
If the possessive is plural to begin with, don't add an S after the apostrophe (do add the apostrophe!). If the possessive is singular, as in there's only one Jess, then add the S after the apostrophe.
But please, don't apostrophe a plural! Trees are plural. NO apostrophe! One rabbit that multiplies becomes two rabbits, NOT two rabbit's. Rabbit's implies one rabbit owns (shows possession of) something, such as the rabbit's hole. Or if a group of rabbits own a hole together, it becomes the rabbits' hole (no S at the end because it's plural AND possessive).
Many new writers think they can rely on their editors (plural, no apostrophe) to catch these things. Unfortunately, I've seen many editors who didn't and the books go out with apostrophe mistakes. For those of you who write professionally, or plan to, don't rely on your editor for grammar knowledge. Learn it the way a mechanic has to learn how to build an engine, with all parts in the right places. That's your job.