What is the difference between zuvor and bisher? Both mean before, but in what context are they used? Are they interchangeable?
Sein Deutsch ist viel besser als bisher.
Sein Deutsch ist viel besser als zuvor.
Bisher means up to this point with this point being the now of the utterance. So bisher is synonymous to bis jetzt although the latter implies a change more strongly in my opinion.
zuvor means before that point with that point being a random point in time that has been established in speech before.
If zuvor just order two activities, it is synonymous with vorher and davor. But since it can also talk about habbits or things of duration, it is the only choice sometimes ... okay... except for bis dahin. which can express that too.