比較have to和must

1)　兩詞都是'必須'的意思，have to 表示客觀的需要， must 表示說話人主觀上的看法，既主觀上的必要。 My brother was very ill, so I had to call the doctor in the middle of the night.　我弟弟病得很厲害，我衹得半夜裡把毉生請來。(客觀上需要做這件事) He said that they must work hard. 他說他們必須努力工作。（主觀上要做這件事）

2)　have to有人稱、數、時態的變化，而must衹有一種形式。但must 可用於間接引語中表示過去的必要或義務。 He had to look after his sister yesterday.

3)　在否定結搆中： 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。