21.4 比较have to和must
 
1) 两词都是'必须'的意思,have to 表示客观的需要, must 表示说话人主观上的看法,既主观上的必要。
  My brother was very ill, so I had to call the doctor in the middle of the night. 我弟弟病得很厉害,我只得半夜里把医生请来。(客观上需要做这件事)
  He said that they must work hard. 他说他们必须努力工作。(主观上要做这件事)

2) have to有人称、数、时态的变化,而must只有一种形式。但must 可用于间接引语中表示过去的必要或义务。
He had to look after his sister yesterday.

3) 在否定结构中: don't have to  表示"不必"
          mustn't    表示"禁止",
   You don't have to tell him about it.  你不一定要把此事告诉他。
   You mustn't tell him about it.    你一定不要把这件事告诉他。





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