Use it if something happened followed by something else. It indicates a continuation. "I learned how to speak properly, then people didn't think I was a moron."
It also is used to refer to a specific moment in time. "My stupidity was blatant then."
|Sideburns were cool then.|
It's used for comparing or contrasting. "Now that I have learned how to speak properly, I feel smarter than before. People who use correct grammar are smarter than those who don't."
Think of it this way: You were using the words incorrectly, then you learned the correct way to use them and you felt better than before.
|Mercury is smaller than Earth.|
Sequential? Then. Comparative? Than.