Than is a conjunction used in comparisons:
Tom is smarter than Bill.
This is more important than you might think.
Is she taller than you?
Yes, she is taller than I.
Technically, you should use the subject pronoun after than (e.g., I), as opposed to the object pronoun (me). However, English speakers commonly use the object pronoun.
Then has a lot of meanings.
1. At that point in time
I wasn’t ready then.
Will you be home at noon? I’ll call you then.
2. Next, afterward
I went to the store, and then to the bank
Do your homework and then go to bed
3. In addition, also, on top of that
He told me he was leaving, and then that I owed him money
It cost $5,000, and then there’s tax too
4. In that case, therefore (often with "if")
If you want to go, then you’ll have to finish your homework.
Then you should eat.
Than is used only in comparisons, so if you’re comparing something use than. If not, then you have to use then. What could be easier than that? :)