'Then' is an adverb which means "at that time," or at a specific time that you are talking about. 'Then' can also mean "next," or "also."
Examples:
1. Let's do our homework, then play.
2. If you can't go, then let me know.
3. Mark will eat his dinner first, then he will eat his dessert.

'Than' is a conjunction and preposition which is use in a sentence that is comparing two things.
Examples:
1. I would rather have chicken for lunch than fish.
2. Ishant is much taller than his brother, Sunny.