1.
Used as a conjunction, yet is similar in meaning to but,
but it has a stronger effect on the reader or listener. Compare:
'The sun was shining and there was no wind, yet it was
unusually cold.'
'The sun was shining and there was no wind, but it was
unusually cold.'
There is perhaps more of a surprise associated with the former
statement.
Note
that you can put and in front of yet when it comes
at the beginning of a clause, but of course this is not possible
with but, so you can say:
'The cyclists were tired and hungry, all but exhausted,
(and) yet they refused to give up in their attempt to finish
the race.'
2.
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?'
3.
For example and for instance are completely interchangeable,
so it is just a matter of personal preference as to which you decide
to use:
'There are a number of rules you must abide by. For instance,
you may not use the swimming pool unsupervised.'
'You have all made silly mistakes on this trip. John, for
example, failed to secure the boat properly and Adam took
the jet ski out when the sea was far too rough.'
