As the title implies,I'm not entirely sure what the differences are between them. As an example:
友達は買い物にいくはずでした。 My friend was supposed to go shopping.
友達は買い物にいったはずです。 My friend must have gone shopping(?)
友達は買い物にいくはずではない/なかった。My friend was not supposed to have gone shopping...(?)
This is my current understanding of it. If anybody could correct or explain the differences to me, I'd appreciate it.