What is the difference between SO and HENCE?
SO as a linking word between two clauses or sentences is similar in meaning to HENCE, though HENCE is much more formal.
Compare:
'Paul didn't have enough money for the train ticket, so he had to go to the cashpoint before he could travel.'
'It is clear to us now that drug abuse can never be beneficial to the user; hence we seem to have got it wrong in suggesting that it may sometimes be acceptable.'
In spoken English, we often begin a sentence with SO, thus making a link with what has been said before:
'We couldn't find the key, so we couldn't open the door.'
'So what did you do?'