Today’s Tuesday Tip comes straight from Grammar Girl herself at www.quickanddirtytips.com.

“Then” has an element of time. For example, it can mean “next” or “at that time.”

“Than” conveys a comparison.

Quick and Dirty Tip: Both “than” and “comparison” have the letter “a” in them, and “then” and “time” both have the letter “e.”

Check out the entire post, including some examples, by clicking here:
http://www.quickanddirtytips.com/education/grammar/then-versus-than