Than is used in comparison statements; He is smarter than I.
Used in statements of preference; I would rather drink than drive.
Used to suggest quantities beyond a specified amount; Read more than the last page.
Then is a time other than now; He was younger then. She will start her new job then.
Next in time, space, or order; First we must study; then we can play.
Suggesting a logical conclusion; If you’ve studied hard, then the exam should be no problem.