unless用作连词，意为“除非”、“如果不”，具有否定意义，因此使用unless引导条件状语从句时，其从句中谓语动词多用肯定式，这时可用if ... not替换；如果unless引导的从句本身是否定形式，unless不能用if ... not替换。例如：[文章版权 禁止转载:考试与评价 http://www.trgroup.com.cn/te/ &!4D.?bRa/ qAw?7 FroG]

    Unless you study hard, you won't go to college. = If you don't study hard, you won't go to college.[文章版权 禁止转载:考试与评价 http://www.trgroup.com.cn/te/ }Z;cdt$,$
.}n!-vJTYFroG]

    You'll do well in this exam unless you don't do your homework.(此句不能用if... not替换)[文章版权 禁止转载:考试与评价 http://www.trgroup.com.cn/te/ ;=]CV
yI]oD;.hVdFroG]