Then or than: which one do you need?

Than introduces a comparison: taller than her brother, more expensive than expected, rather than wait. Then indicates time, sequence, or consequence: we ate, then left; if it rains, then we cancel; back then. The test is to ask whether two things are being compared. If yes, than. If the sentence is about order or timing, then. There is no context in which they are interchangeable.

The confusion is phonetic. In rapid speech both words reduce to nearly the same unstressed vowel, so the ear provides no help and spelling has to be learned as a rule rather than heard. The error runs both ways but appears most often as then where than belongs, as in better then nothing. Spellcheck does not catch it because both are valid words.

Two adjacent traps of the same type worth checking. Other than is a comparison and takes than, while the phrase rather than is also comparison. And the informal phrase different than is widely used in American English and is accepted by most dictionaries despite traditional preference for different from, so that one is a style choice rather than an error in the way then for than is.