5) Words to write right: its, it’s, who’s, whose, their, there, they’re
The saying used to be, “On the Internet, no one knows if you’re a dog.” (Cats are obvious, though; they always fall asleep on the keyboard.)

Likewise, when you’re talking, no one can tell if you screw up whose and who’s, its and it’s, and there, they’re and their. But when you’re writing, watch out. Screw these up and you’ll look like a fool.

But don’t despair. Or, as grandmothers say, “There, there.” You can expand your mind on this front by understanding contractions.

If what you’re really saying is “it is,” “there is,” or “who is,” then use the apostrophe version. So, it’s, there’s, and who’s.

Use its, whose, and theirs for possessives. How do you remember this? Just remember that possessive pronouns have everything–except apostrophes.

Likewise, I have another goofy little trick for keeping their straight from they’re and there. Their is a possessive. It has an i in it. I like owning things. Therefore, the one to use there is their.

There’s, meanwhile, comes with a warning. There’s is short for there is. “There’s a fly in my soup” is correct, if disgusting. But “There’s flies in my soup” is incorrect and even more disgusting. So be careful there. Even one of my smartest high school students–an award-winning writer–made this mistake in a recent piece she wrote for me.

My bet is this is a common mistake because it’s easier to say “there’s” than “there’re,” which is the correct verb form whenever you’re dealing with more than one fly. Those two r sounds in a row don’t exactly trip off the tongue. While you can get away with it in speech, the error will be more obvious in writing. So beware.

And now, for two final entrants into the write-it right category: loose and lose.

This one’s easy. Loose rhymes with goose. And as they said in Buffy the Vampire Slayer, “How loose is your goose? Your goose is totally loose”–whatever that means.

Meanwhile, lose is a loser. It has lost its other o. And if you can remember that, you’ll be a winner–grammatically speaking, at least.

4) Dangerous malapropisms: Say what?

3) I seem to have misplaced my modifier

2) Avoid apostrophe catastrophes

1) Be agreeable

This list has been severely abridged. To read the full list and full descriptions, view the original post at it’s source:
5 Embarrassing Grammatical Mistakes (Encarta)