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.)