1. "Than" is used to indicate a comparison.
For example: She was smarter than you.
("Than" is used with a comparison.)

2. "Then" relates to time or means "in that case.
For example: I was fitter then.
(Here, "then" relates to a past time.)

3. The word "than" introduces a comparison. It is most often seen with comparatives and words like "more," "less," and "fewer."
For example: Craig is smarter than Paul.
("Smarter" is a comparative.)

4. The word "then" usually relates to time. It is most commonly used as an adverb. It has the following meanings: Subsequently or afterwards
For example: Go to the traffic lights, then turn right.