"needn't" 表示不必要或不需要做某事，是一种选择性的动词；而 "mustn't" 表示禁止或不允许做某事，是一种强制性的动词。
大家可以先看下面的表格了解一下 needn't和mustn't 吧：
"needn't" 和 "mustn't" 的区别：
区别1：意义和用法
"needn't" 表示某件事情不是必要的，没有必要去做，但是可以选择去做。它是 "need" 的缩写形式，意为 "不必要"。
例子1: You needn't worry about the test. It's quite easy. (你不必为考试担心，它很简单。)
例子2: She needn't attend the meeting if she doesn't want to. (她如果不想去，就不必参加会议。)
"mustn't" 表示某件事情是禁止的，不允许去做。它是 "must not" 的缩写形式，意为 "不许，禁止"。
例子1: You mustn't use your phone while driving. (开车时不允许使用手机。)
例子2: Students mustn't bring electronic devices into the examination room. (学生们不得将电子设备带入考场。)
区别2：情态动词的强制程度
"needn't" 表示轻微的建议或建议，不像 "must" 或 "have to" 那样强制执行。
例子1: You needn't finish all the food on your plate if you're full. (如果吃饱了，你不必把盘子里的食物都吃完。)
例子2: He needn't apologize for the mistake; it wasn't a big deal. (他不必为错误道歉，那不是什么大不了的事情。)
"mustn't" 表示严格的禁止，强调必须遵守的规则或规定。
例子1: You mustn't smoke in this area; it's a non-smoking zone. (在这个区域禁止吸烟，这是无烟区。)
例子2: Employees mustn't disclose confidential information to outsiders. (员工不得向外部透露机密信息。)
区别3：否定意义
"needn't" 否定的是需要做某事的必要性，表示没有必要去做。
例子1: We needn't hurry; the movie starts in an hour. (我们不必着急，电影一个小时后开始。)
例子2: She needn't buy a new dress for the party; her old one is just fine. (她不必为聚会买一件新裙子，她的旧的还不错。)
"mustn't" 否定的是行为的允许性，表示不允许去做。
例子1: You mustn't forget to lock the door before leaving the house. (离开房子前不许忘记锁门。)
例子2: Visitors mustn't take photographs in the museum. (参观者在博物馆内不得拍照。)
区别4：建议和规定
"needn't" 常用于给出友好的建议或提供可选的选择。
例子1: You needn't bring anything to the party; your presence is enough. (你不必给聚会带什么，你的到场就足够了。)
例子2: Students needn't wear a uniform, but it's recommended. (学生们不必穿制服，但建议这样做。)
"mustn't" 常用于明确规定或规则，强调不允许的行为。
例子1: In this library, you mustn't talk loudly. (在这个图书馆里，不允许大声说话。)
例子2: Employees mustn't access personal emails during working hours. (员工在工作时间内不得查看个人电子邮件。)