You haven't lost your library book,have you? ----I hope not.
你没有丢掉书本,是吗?回答不应该是 I hope so 吗?
你的理解过于狭隘了。so和not都可以作为分句替代词,替代一个分句。二者的区别是,so替代的是肯定分句,not替代的是否定分句。例如:
Do you think it will rain tomorrow?
I think so. = I think it will rain tomorrow.
I think not. = I think it will not rain tomorrow. = I don't think it will rain tomorrow.
所以,你的句子中,I hope not. = I hope you haven't lost the book.
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