must 和 have to + 动词原形都表示这是必须的，你需要去做。（must 往往是自己觉得必须；have to 往往是因为有规定。）说过去必须做的事，用 had to：Yesterday I had to work late。
must 和 have to：这是必须的
I must finish this today.
You have to wear a seatbelt in the car.
She has to study for her exam.
试一试: It's the law: you ___ wear a seatbelt. (have to / have / must to) 答案: have to — 因为规定所以必须做，用 have to + 动词原形：have to wear。"Have" 单独用漏掉了 to，而 "must to" 永远不接 to（就是 must wear）。
mustn't：这是不允许的
Mustn't + 动词原形表示禁止，违反规定。它跟 "don't have to" 不是一回事。
You mustn't smoke in here. (it is forbidden)
You mustn't park there.
Students mustn't use phones in the exam.
试一试: 咖啡馆里挂着 NO SMOKING 的牌子。在这里吸烟是违反规定的。 哪个更符合不被允许的情况？ (You mustn't smoke here. / You don't have to smoke here.) 答案: You mustn't smoke here. — 吸烟违反规定，所以是禁止的：You mustn't smoke。"You don't have to smoke" 的意思是抽不抽烟是你的自由选择，而 NO SMOKING 的标志表达的不是这个意思。
don't have to：你可以自己选
Don't have to / doesn't have to + 动词原形表示没有必要，你可以自己选择。它不是禁止的意思。
You don't have to come if you're tired. (it's your choice)
She doesn't have to work on Sundays.
We don't have to wear a uniform here.
试一试: 这个派对就是为了好玩。Amina 希望 Lars 能来，但来不来完全由他自己决定。 哪个更符合可选的情况？ (You don't have to come. / You mustn't come.) 答案: You don't have to come. — 来不来完全是他自己的选择，不是必须的，所以用 don't have to：You don't have to come。"You mustn't come" 会表示他被禁止来，这跟友好邀请的本意正好相反。
试一试: 在博物馆里，触摸画作是违反规定的。 在博物馆里，拍照是允许的，但拍不拍完全由你自己决定。 把每一句配对起来。哪个符合哪条规定？ 答案: 在博物馆里，触摸画作是违反规定的。 → 1 → You mustn't touch the paintings. · 2 → You don't have to take photos. 在博物馆里，拍照是允许的，但拍不拍完全由你自己决定。 → 1 → You don't have to touch the paintings. · 2 → You mustn't take photos. — 触摸是违反规定的 → mustn't（禁止）。拍照是可以自己选的 → don't have to（可选）。mustn't 表示不允许，don't have to 表示你可以自己选。