This document discusses the past tense modal "should have" and how it is used to express regret over past events that did not happen or actions that were not taken. Should have implies something should have been done differently in the past. It is used to speculate about past alternatives and to acknowledge mistakes. Examples are given showing how should have is used to express that someone should have studied more, called first, or woken up earlier. It can also be used in apologies to acknowledge something that should have been done differently in the past.