'Then' means "at that time", "soon after" or to indicate a consequence. "I turn 40 next week. Then I'll be middle-aged." "First add water, then add salt" "If 2+2 is 4, then adding 1 to that makes 5" 'Than' is used in comparisons, or to indicate a difference or deviation. "You are better than me" "Anybody else than me" "Barcelona won all games this season, other than the one against Llevante." "Rather than play Messi, Barcelona decided to let him rest."

You will see 'then' used instead of 'than' all over the internet in comment sections on blogs for example. It's misuse is becoming more commonplace rapidly. It's because many native speakers can't spell properly so they use then instead of than because they don't understand the difference.

Then is more easily understood as meaning next, except in English colloquial use were "i'll see you then" = I will see you later or whenever I see or whenever I arrive etc. "ok then" = agreement Than is used to compare things.