Then" is a measure of time, as in "back then, when I was younger".
"Than" is a measure of more or less, as in "His truck is bigger than mine".
That's the easiest way I know how to explain it. Hope it helps.
Oooh, oooh, I got it!....it's the 'e' and the 'a'.....
Then means "afterwards" or precedes the second clause of an if-then statement.
I went to the store, then I went home.
If I spend too much, then I won't have any money left.
Than is a comparative.
I am taller than you.