Of all the mistakes I see in English spelling/grammar (and there’s no shortage of them) this is the one that bugs me the most. I don’t know why. Maybe because it’s so common. Or because it’s one of the first rules I learned.
I was seven years old. Our teacher took a whole lesson to explain and I’ve never forgotten it. Here’s the rule as she explained it:
IT’S usually stands for IT IS. Sometimes it stands for IT HAS.
ITS is correct if you can insert the word OWN after it.
That’s all there is to it. So why do people get it wrong so often?
Because ITS without an apostrophe is possessive. It means belonging to it. And the possessive form with a noun is spelt with an apostrophe, as in MIRIAM’S BUGBEAR. So some people’s logic tells them that ITS meaning belonging to it also requires an apostrophe. It doesn’t. Logic is not always a sensible attribute to use when writing English.
Another reason is that English grammar is no longer taught as it used to be. That’s considered unnecessary these days. Grrr!