情态动词主要说明说话者对某一行为的态度和观点，包括“揣测、判断、需要、能力、可能、许可和各种意愿”等。

     can, could表示“能力”；can, could, may, might表示“许可”；must, have to, should, ought to表示“义务、责任”；shall, will, would表示“意愿、征求意见”；情态动词表示推测。

     情态动词的两个特征：词义不完整，不能单独做谓语；没有人称和数的变化（have to例外）。

     学习情态动词的难点：每个情态动词都有多个意义，要根据句意和语境来判断；有些特例和规定要着重理解。具体要求：①词义辨析；②情景中的运用；③情态动词+have+过去分词；④情态动词习惯用法；⑤反意疑问句。

一、can, could

1. 表示能力
—Can you drive? —Yes, I can.
What he said could not be heard by the students.
2. 表示（征求）许可或客观条件允许
Nobody can smoke here.
—Could I have a look at another pair? —Of course you can./No, I’m afraid you can’t.
3. 表示可能性、后接“be (get, seem, become)＋形容词”表示“有时候会”。
Anyone can make mistakes.
London can be very cold.
4. 表示说话人的推测、怀疑、惊奇、或不肯定，多用于否定句、疑问句和感叹句。could可以用于肯定句。
The news can not be true.
The essay can’t be finished in such a short time.
Can this be true?
How can/could you be so careless!
The injured could be in hospital now.
—Or they could be in one of those empty cottages near the pine trees…
—They can’t be there. People would have looked there first, I’m sure.
5. 如果对过去情况推测判断的句子是疑问句和否定句，用can／could和can’t／couldn’t．(may/ might have done也用于对已经发生过的情况的肯定推测判断，语气稍弱，意思是“可能，或许…”,一般不用于疑问句)。例如：
He can’t have gone abroad; I saw him an hour ago. 他不可能去国外，我一个小时前还见到他呢。
—Can she have passed the exam? — No, she can’t have. She didn’t work hard.
他通过考试了吗？不，他不可能通过考试。他学习不努力。
—They might have fallen behind and taken the wrong way.
—Maybe. Or they could have gone exploring on their own and lost their way back to the group.