The document discusses using modal verbs to talk about past events and deductions. It specifically focuses on using "must have" to express that something likely happened based on evidence from a past event, with a 90% certainty. Examples are given such as "My teacher must have been in a traffic jam" based on them arriving late. It also discusses using "should have" to express regrets about past events not happening or happening, with examples like "I should have studied harder" for failing an exam.