Firstly, than is a conjunction used mainly in comparisons. Then is primarily an adverb usually used place actions in time. For example: First, I went to the post office, then I did the shopping, then I went home. In distinguishing between the two words, it is useful to remember that than has no one-word synonyms. Promotion is cheaper than advertising.The word then most often relates to time. Then can be used as enumeration, as well.
