Then and Than

Then always indicates a relationship in time.  Something happened, and then something else happened after that.  When you use "if" with "then" to show a cause-and-effect relationship, the effect still comes after the cause.  For example, "If you help me, then I will be grateful."

Than is used for comparisons.  Brazil is bigger than Portugal.  Mice are smaller than cats.  Chess is more complex than checkers.  Even when talking about time, use "than" for comparisons.  Autumn is later than summer.