This document discusses the use of modal verbs to express possibility and certainty in the present and past tense. It explains how to use might, may, could, must, have/has to, have/has got to in the present and past for guessing or certainty. It provides examples of how to use modal verbs with the present perfect tense to talk about past time events. Finally, it discusses using should + present perfect and ought to + present perfect to express regret about the past.