Use than to make a comparison. Use then when referring to time.
- The quiz was harder than I had expected.
- I answered two questions and then got stuck.
- "Look here, Jimmy. You misspelled culpable. And you’re confusing then and than. T-h-e-n is an adverb used to divide and measure time. 'Detective McNulty makes a mess, and then he has to clean it up.' Not to be confused with t-h-a-n, which is most commonly used after a comparative adjective or adverb, as in: 'Rhonda is smarter than Jimmy.'"
(Judge Daniel Phelan to Detective Jimmy McNulty in the episode “One Arrest," The Wire, 2002)
- "[T]hen for than is an error much commoner than highbrows seem to think: it is not merely the illiterate who fall into it. The reason is not that, several centuries ago, than and then were spellings and pronunciations frequently interchanged, but that, where than bears no stress and is spoken very rapidly and lightly, it tends to approximate to then."
(Eric Patridge, The Wordsworth Book of Usage and Abusage, rev. 1995)
(a) I filled out a form and _____ waited in the lobby.
(b) I had to wait much longer _____ you did.