i don't think this question will stay, but I'll have a go at it.

then and than do not sound the same when spoken, they sound similar, but not the same. the 'e' in 'then' is a more closed sound than the 'a' in 'than'.

you might want to compare other words with the same sounds:

• ben, ban
• send, sand
• lend, land

'then' tends to be used when talking about sequences. "i do this, then i do that", it can also be logical sequences: "if you rob a bank, then you will go to jail"

'than' tends to be used more for comparisons: "I'd rather have a nice car than a small payrise", "there's nothing more satisfying than spending the night with your friends".

although I'm not a linguist so I'm probably missing a lot of uses.