▲根据外文出版社与商务印书馆联合出版的《英语语法系列：连词》：

Unless 与 if…not 的意义并不完全相同。Unless 强调例外的事件，所以准确地说，它的意义为“除非”。当条件与已知事实相反时，必须用 if…not, 不用unless。

It would have been comic if it had not been so frustrating. 即使这事不是那么令人感到灰心受挫，也是使人感到好笑的。

If the body had not been on the path it would have been ignored. 如果尸体当时不在路上，就会无人过问。

误：It would have been comic unless it had been so frustrating.  

误：Unless the body had been on the path it would have been ignored.  

▲但是，unless 有时可引出一个过去未曾实现的假想法情况，对前面主句进行补充说明(此时 unless 必须置于主句之后，且 unless 前面常用破折号而不用逗号)。比较：

◎I couldn’t have got to the meeting on time—unless I had caught an earlier train. 我不可能及时赶到会场，除非我赶上了较早一班火车。(事实是“我”没有及时到会)

此句若将 unless 改为 if…not，则成为：

◎I couldn’t have got there If I hadn’t caught an earlier train. 如果我没赶上较早一班火车，我就不能到达那儿了。(事实是“我”及时到会了)