Step 1 lead-in
      Step 2 learning aims
1. To learn the general meanings and functions of modal verbs2. To learn how to use several common modal verbs
What is modal verb?What are the similar usages of modal verbs?
        Step 3 self-study
情态动词是表达能力,义务,必须,猜测等说话人旳语气或情态旳动词.
情态动词不能单独做谓语，除ought和have外，背面只能接不带to旳不定式。