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。
例句:
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。
例句:
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"。
例句:
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。