What is the difference between ばよかったです and べき?

Question

What is the difference between ばよかったです and べき?

あのバスに乗ればよかったです。
I should have taken that bus.

あのバスに乗るべきだった。
I should have taken that bus.

このバスに乗れるはずじゃなかった。
I wasn’t supposed to get on this bus.

簡単なはずです。
It should be easy.

Answer 1

ばよかった - "should have" in the sense "I wish I had" or "it would have been better if I had"

べき - "should have" in the sense of "ought to". There is some expectation from the outside, some other people, or society as a whole that you should better do it.

はず - in contrast to べき, the expectation or assumption here comes from yourself, prior experience etc.

Answer 2

ばよかった - means "I wish I had" , you're regret about something

for example: "彼女と別れればよかった" = "I with i had broke up with my girlfriend"

べき - means "you're supposed to do something " or "you're responsible for doing sth"

for example: "今日中にすべき仕事"　= "The work have to be done today"

Hope this helps