To my eyes, the words its and it’s cause the greatest confusion. I know I have written about this before, but in case you need a quick refresher, here goes:
Its is a possessive pronoun: The dog wagged its tail. The tail belongs to the dog. No possessive pronoun ever has an apostrophe: hers, his, our, theirs—see? No apostrophes. Its is a possessive pronoun; therefore, no apostrophe, ever.
It’s is a contraction. It means either it is or it has:
It’s expected to rain later today. (Substitute it is.)
It’s been a long time since Southern California had a good rain. (Substitute it has.)
When you are thinking about using it’s, the one with the apostrophe, see if you can substitute it is or it has. If you can’t, you want the possessive form, its.
Two other words that cause serious problems are who’s and whose. This distinction is just as easy:
Who’s is a contraction, meaning who is or who has:
Who’s going to run for committee treasurer? (Substitute who is.)
“Who’s been sleeping in my bed,” growled Daddy Bear. (He means who has.)
Whose is a pronoun showing ownership: Whose pen is this? If you can’t substitute who is or who has, you want the possessive form, whose.