"Would you be sad if she does not come?"
Translation:Skulle du bli ledsen om hon inte kommer?
You mean why is it "Bli" instead of "Vara"?
Because they will "Become" sad.
While we would use "Be", Swedish uses "Bli" instead of "Vara" when something is changing from one state to another.
Vore is the simple past form of to be, and would translate roughly as Were you sad if she does not come, which isn't acceptable in either the English or Swedish. To make it conditional, you might be inclined to think skulle vara (would you be) would work nicely, but as a group of three comments above from Berniebud through Lundgren8 have indicated, Swedish in practice uses the equivalent of Would you become (skulle du bli) rather than the less discriminating Would you be of English for this sort of situation.