"Then" is used in an "if-then" scenario, to dictate chronological order. In other words, "If you jump off, then you will fall," or "he was attacked and then retreated."
"Than" is used for comparisons. In other words, "The egg was bigger than the pea," or "I'm stronger than you."
It's normally used to explain a difficult choice to an audience of friends ex. when you are transferred abroad or to another city or state,after having worked in the same place for a long time.
