It’s= it is. Its=possessive. I think some of people’s confusion on this one stems from the fact that when you make a proper noun possessive, you add an apostrophe (Sally’s dog) but with a pronoun you don’t. Its is the possessive form. It’s is a contraction for it is. What’s a contraction, you ask? Lin
I don’t know why this one is so frequently butchered, but it is. This is all you need to know: There indicates location as in “the car is over there.” They’re is a contraction for they are, as in “They’re good students.” Their is possessive: “their birthday cake.”
Oh man. This one makes me nuts. Saying “I” all the time does not make you sound smart, and it’s not always correct. Here’s an easy rule: whichever one you’d use if you were alone, use it if you’re with someone else too. “I am going to the store,” so “Joe and I are going to the store” and “That cake was for me!” so “That cake was for Jane and me!
4. Apostrophe catastrophes
Apostrophes! Okay, here’s the breakdown: Apostrophes are for making contractions (Mary’s going to the mall) and showing possession (Dave’s dog). They are not for making nouns plural, unless (there’s a lot of those in our lovely bastard language) it’s to clarify something–CD’s is okay, for instance, because just CDs might look a little weird or confusing. One more thing: those people with family address labels that say “The Smith’s”? They’re idiots. You can talk about meeting the Smiths, or going to the Smith’s house, but Smith’s is just an eyesore.
I’m going to do an entire post about commas later because…well, people can’t use commas. But I’ll say this: commas are a lot like drugs, in the sense that some people abuse them and a few people should really start using them more often. The next time you’re not sure where to put a comma, read your sentence out loud. Hear that spot where you took a natural pause to take a breath or to start a new thought? That’s where the comma goes.