When can I use "must have been?" Is it different from "should have been?"
Answer from a Native speaker
Yes, "must have been" is different from "should have been." "Must have been" means to draw a conclusion or make a judgment about the past. Whereas "should have been" is when you expect or expected something to happen that probably didn't happen. Ex: The food should have been delivered by now, but it isn't. Ex: The food must have been delivered by now at the party. We'll eat when we arrive. Ex: She must have been scared when the incident happened.