The document discusses different modal verbs used to express obligation, advice, permission, deduction, and ability in English. It explains: 1) "Have to" and "must" both express obligation, with "have to" used more for general rules and "must" for personal obligations. Their negatives have different meanings. 2) "Should" and "ought to" are used to give advice or make recommendations. 3) "Be allowed to" expresses having permission to do something. 4) "Must", "may/might", and "can't" are used to make deductions with different levels of certainty. 5) "Can", "could", and "be able to"