"我必须走了"是 I must go 还是 I have to go？两个都对，但语气差不少。
must：说话人的命令
出于说话人自己的判断或规定，"必须、一定"：
- You must finish homework first.（家长/老师说的）
- I must stop smoking.（我自己下决心）
have to：客观逼的
外部情况让你"不得不"：
- I have to get up early tomorrow. 明天不得不早起。（要赶车）
- She has to wear glasses. 她不得不戴眼镜。（视力差）
过去式和将来式
must 没有过去式，过去和将来要用 have to 顶替：
- I had to work last Sunday.
- You will have to pay.
mustn't 和 don't have to 天壤之别
- mustn't = 禁止、千万别：You mustn't smoke here. 这儿禁烟。
- don't have to = 不必、没必要：You don't have to come. 你不用来。
一个"千万别"，一个"随你便"，别搞反。
肯定推测
must 还能表"肯定是"：
- The light is on. He must be home. 灯亮着，他肯定在家。
口诀：must 主观看必须，have to 被现实逼；mustn't 禁止 don't have to 随意。
