This document discusses the use of modal verbs to express possibility and probability in English. It explains that "could" is used to express present or future possibility, while "could have" is used for past possibility. "Must" expresses certainty and "can't" expresses impossibility. "Must have" and "can't have" are used for past certainty and impossibility. "May" and "might" express present possibility, while "may have" and "might have" are used for past possibility. "Could", "might", and "may" are similar in meaning, while "couldn't" differs. Finally, "should" and "ought to" are used to express obligations or recommendations.