It might be because the words are pronounced similarly that then and than are often confused by English learners. Let's take a look at the difference.
Then is used either as a time expression, as a consequence or with a sequence of events.
As a time expression - Sorry I missed your call last night, I was already sleeping then.
As a consequence/result - Eat now, then you won't be so hungry later.
As a sequence marker - First boil the water, then add the noodles.
Than is not related to time, it is a conjunction. Than is used in the comparative form i.e. in comparative statements when comparing two things.
You are older than I am.
She was later than usual.
That was much better than I was expecting.
A simple trick to help you remember the difference is that then is a time word like when. Both of these words are have an e in them. If you are talking about time, use the e word then.
Now decide if these sentences need then or than: