Here is an English usage worksheet on differentiating than and then in prose and speech. This relatively minor area of usage is nonetheless sufficiently thorny to have warranted a surprisingly lengthy exchange on it between two characters on The Wire. As the state’s attorney explains to the detectives seeking a court order for a wiretap (as does this worksheet), than is a conjunction and then is an adverb.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.