Here’s another situation that is changed mightily by one letter. I not only see this error often in manuscripts but I hear it in the spoken word as well. There is a difference!
Then – used to indicate what happened, happens next, or what should be done next.
Than – can be either a conjunction or a preposition. Most commonly used with comparative adjectives and comparative adverbs.
Examples: If the puppy grows bigger than her cage, then you’ll need to buy her a new one. (In the first half of the sentence, we are comparing the size of the pup versus her cage. In the second half of the sentence, an action will take place if the condition exists.) Another: We can move the table over there and then put the couch here, but that will only work if the couch is shorter than the windowsill. (Again, the couch will only be moved after the first action is completed and in the second part of the sentence, we are comparing the height of the couch relative to the windowsill.)
It’s easy to figure out whether to use then or than just by thinking (ahead) how the word will be used in your sentence.