Than is for comparison, then is for time. That's how they taught us in school. For than that holds, then has a bit of a vernacular life of its own, like in "What's this, then?".
I seem to confuse you even more now with "than" and "then". I always found it mind-boggling how you don't know the word "than" and how to use it. Better than, bigger than, more beautiful than and so on...