The students must study hard,mustn't they
反义疑问要根据前面的句子来确定,前面用must,后面当然要用mustn't.
但疑问句的回答有所不同：
用need来提问时,其一般疑问句的答语,肯定用must,否定用needn't.例如：
Need I go with her 我需要和他一起去吗?
Yes,you must.是的,你需要去.
No,you needn't.不,你不必去.
而用must来提问时,肯定用must,否定的用needn't或者don't have to.
例如：
Must I clean the dining room at once?我必须马上打扫餐厅吗?
Yes,you must.是的.
No,you needn't.不必马上打扫.（No,you don't have to.）