Than can be used as a conjunction and as a preposition in English.
We often use it to compare things.
For example: "Laika is smaller than Sam."
For example: "When I looked at my money, I had less than I thought I had."
Then can be used as an adjective or an adverb. It is used to describe time in the past or the future.
For example: "They say machines will do all the work in the future, but I think I'll be retired by then."
If you still get them confused think "When = Then".