unless & if…not 是引导否定条件从句(negative conditional clause)的同义连接词。由于 unless 本身包含有否定意思,故常译作“如果不”,“除非…否则”,而且不少语法书都将这两个连接词等同起来,并且认为两者都可以互换使用。这样一来似乎它们之间可以划等号了,其实也不尽然。在实际应用时,二者仍存在着细微的而不容忽视的差别,但究竟这两个否定连接词在用法上有哪些不同之处呢本文仅就其不能换用的情况作一介绍: Unless & if...not is a synonymous conjunction that leads to a negative conditional clause. Because unless itself contains negative meanings, it is often translated as “if not”, “unless... Otherwise”, and many grammar books equate these two connectives and think that both can be used interchangeably. As a result, it seems that there can be an equal sign between them, but it is not always true. In practical applications, there are still subtle differences that cannot be ignored. However, what are the differences in usage of these two negative connectives?