I see this mistake often, and I often have to think about the difference between the two. So I thought I'd share.

Than versus Then.

Than is a conjunction or preposition used in unequal comparisons:
(The taco was tastier than the burrito.)

Then is (usually) an adverb indicating time or consequence:
(I bit into the taco. It was then I realized, the taco was tastier than the burrito.)