must / have to
必须；must 是主观，have to 是客观。
must do／have to do
要点
- must 表示说话人主观认为「必须」。
- have to 表示客观条件要求「不得不」。
- must 的否定 mustn't 表示「禁止、不许」。
- 「不必」要说 don't have to 或 needn't，不能用 mustn't。
- must 缺将来式也缺过去式：说"将会不得不"借 will have to，说"当时不得不"用 had to（I had to leave early yesterday.）—— 两个空缺都由 have to 顶上。
- 否定与疑问的走法完全不同：must 直接提前或加 not（Must I go? ／ I must not…），have to 要借 do（Do you have to go? ／ I don't have to go.）—— 这是两者在句法上最大的分岔。
- must 还有一层"偏偏"：Why must you complain about everything?（什么事你都要挑一句）—— 这时说的不是义务，是说话人的不耐烦。
例句
You must be on time.
你必须准时。
We have to wear uniforms.
我们必须穿校服。
She has to help her parents.
她不得不帮她父母。
I must prepare for the exam.
我必须为考试做准备。
Do you have to pay for the parking?
停车要付钱吗？
They will have to wait until the bus comes.
他们只好等公交车来。
易错提示
- mustn't = 不许；don't have to = 不必 —— 意思完全相反。
- must 也能表示肯定的推测：It must be Tom's.（一定是汤姆的）
- "不许"这一层有三种软硬：mustn't 最正式最硬，can't 说规矩（You can't park here.），Don't…! 是当场命令 —— 别把它们都写成 mustn't。
- 回答 Must I …? 时"不必"要说 needn't 或 don't have to；答 mustn't 意思就翻成"不许"了 —— 这一问一答是两个不同的词。