Then has two meanings.
1. Then usually refers to time, meaning "at that time."
Example of then meaning "at that time"
2. Then can also be used as a transition to mean "in that case" or "therefore."
Example of then meaning "in that case" or "therefore"
Than is used only when comparing items. (Then is never used in comparisons.)
Examples of than used to compare items
Hint: Both than and comparing are spelled with an a.
Therefore, use than when you are comparing things.
Link to exercises on then / than.