It's vs. Its
"It's" is a contraction, which stands for "it is" or "it has." Examples:
It's a beautiful day out. (It is a beautiful day out.)
It's been raining for three days straight. (It has been raining for three days straight.)
It's a very small boogar. (It is a very small boogar.)
When you don't know whether to use it's or its, give it the "it is/it has" test. Say the sentence to yourself and if you can't replace "it's" with "it is" or "it has," then it's possessive. No apostrophe.
Its is possessive, which I know sounds weird, but just pretend like it works. It would probably be more confusing if it was both possessive and a contraction, after all. Examples:
Our family tree has its roots in Scotland.
The camera has its pictures stored digitally.
The alarm clock has its own way of telling time.
In each of these instances, again try the "it is/it has" test and it won't work. This is the most common error I keep seeing repeatedly so I'm hoping this helps some non-grammar nerds.
Then vs. Than
I didn't realize this was such a hard one, but it's the next most frequent error I see. "Then" deals with time. This and then that happened.
Now and then I feel exhausted.
Someone tried to steal my car, and then my husband jumped on its hood.
"Than" deals with comparison.
This was bigger than that.
I feel more exhausted than I did yesterday.
Someone tried to steal my car, but my husband was bigger than she was.
I hope this helps. If folks are interested, I might do some more on common mistakes, particularly commas versus semicolons.