This document discusses the use of modal verbs in English to express ability, possibility, obligation, prohibition, deduction, and absence of obligation. It explains that "can" expresses ability in the present, "could" expresses ability in the past, "must" and "have to" express obligation depending on whether it is a personal opinion or rule/law, "had to" expresses past obligation, "mustn't" expresses prohibition, "don't have to" and "didn't have to" express absence of obligation, and "must" and "can't" express logical deduction or certainty. Possibility is expressed using "can", "could", "may", and "might".