情态动词的推测性用法
    1.Possibility(may/might/can/could)
         a.在疑问句中表示“可能”，须用can而不用may.
         b.在肯定陈述句中，用can表示“可能”与用may往往产生含义上的微妙差别：用表示“可能”往往指逻辑上的；用表事实上的。
    Mr Reed is in poor health. He can be ill at any time.
         Mr Reed looks pale.He may be ill.
     c.用can表“可能”，较多地用于否定句和疑问句，could不受此限。
     If you don't have a guide, you could lose your way.
         It cna't/couldn't be true.
         Can/Could the news be true?
     d.表过去的“可能”可用“may/might+不定式完成体或不定完成进行体”，也可用“can/could+不定式完成体”。
    She may/might have been expressing her true feelings at that moment.
         She can't/couldn't have missed the train.
     e.用“might/could+不定式完成体”有式可以表本来可能发生但没有发生，或本来可能完成却没有完成的动作。
     You might have killed yourself.
         I could have reported you.