My brother was very ill, so I had to call the doctor in the middle of the night. 我弟弟病得很厲害，我只得半夜里把醫(yī)生請來。(客觀上需要做這件事)
He said that they must work hard. 他說他們必須努力。(主觀上要做這件事)
2) have to有人稱、數(shù)、時(shí)態(tài)的變化，而must只有一種形式。但must 可用于間接引語中表示過去的必要或義務(wù)。
He had to look after his sister yesterday。
3) 在否定結(jié)構(gòu)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訴他。
You mustn't tell him about it. 你一定不要把這件事告訴他。
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