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.