"Then" is usually related to time, and is also used in "if... then..." forms. First I had breakfast, then I went to work. If you are sick, then you can stay home from school. "Than" is mainly used to compare things: bigger than, easier than, more than. A lion is bigger than a skunk. Math is easier than Physics. You make more money than I do.

For your reference: http://www.grammar-monster.com/easily_confused/than_then.htm