例：The wall has been like this since being damaged by a bomb. 自从被一枚炸弹炸毁之后，这堵墙就一直是这个样子。
这个句子显然是 The wall has been like this since it was damaged by a bomb 的简化，为什么不能像when、if等引导的状语从句那样省略“主语+be动词”成 since damaged，而是since being damaged？
原因如下：
when、if与since的不同之处在于：when、if只是引导词，since除了是引导词，也是介词。
