I was taught that, at least, 'therefore' and 'so' and can be used interchangeably, one being informal, the other formal. But, even when written, replacing 'so' with 'therefore' doesn't seem correct.
I was tired so I fell asleep.
I was tired therefore I fell asleep.
Am I even allowed to use therefore as a conjunction here? The dictionary says I can, but it would seem more suitable to say:
I was tired and therefore I fell asleep.
I realise hence and thus (and even ergo) are rarely used, but where do they fit into this?