You must come to school at seven.

问题描述:

You must come to school at seven.

You needn't come to school at seven.
原句为:你必须七点钟到学校来.
否定顺为:你不必在七点钟到学校来.
几个情态动词的否定一般就是直接加not的否定,如should/could/need.
must是个比较特殊的例子,其否定形式不是直接加否定,而是变成need再加否定.