Clear rules determine when these two words are used. They are in no way interchangeable.
Then: Used for time: “Then we had ice cream; now we have ice cream soup.”
Then: Used for consequence, with if: “If it melts, then we’ll have soup.”
Than: Used for comparisons only, such as finer: “Nothing could be finer than to be in Carolina.”
Again, only with comparisons, such as all the other options: “Other than waiting, we had nothing to do.”