情態動詞的推測性用法
    1.Possibility(may/might/can/could)
         a.在疑問句中表示“可能”，須用can而不用may.
         b.在肯定陳述句中，用can表示“可能”與用may往往產生含義上的微妙差別：用表示“可能”往往指邏輯上的；用表事實上的。
    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”，較多地用于否定句和疑問句，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.表過去的“可能”可用“may/might+不定式完成體或不定完成進行體”，也可用“can/could+不定式完成體”。
    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+不定式完成體”有式可以表本來可能發生但沒有發生，或本來可能完成卻沒有完成的動作。
     You might have killed yourself.
         I could have reported you.