SAT一道写作/语法/改错题 The bus would not have had to take the long d

虚拟语气掌握主从句谓语动词的规范搭配：
主句 从句
　　与现在事实相反 would/情态动词过去式+do were (不分人称)/did
　　与过去事实相反 would/情态动词过去式+have done had done
　　与将来事实相反 would/情态动词过去式+do should do / were to do
这句话的意思是“假如那座桥在大雪之后不危险,那公交车也就不要绕远路而直接选择主干道了” 这就是个典型的虚拟语气.
主句与过去的事实相反,公交车肯定是绕道了的,那么从句应该就是if the bridge would not have had become treacherous in the aftermath of an ice storm.

if the bridge was not become

虚拟语气的运用分三种情况，分别是对过去的事实、现在的事实以及将来的事实进行相反的假设，通过观察该句中主句谓语的构成我们可以发现，该句是对过去的事实进行的假设，即：
主句：would/could等+ have done; if从句中应为：had done。
所以答案C是正确的，应该改为：had not become...

错在虚拟条件句的条件部分。提炼句子主干如下：
The bus would not have had to take the long detour if the bridge didn't become treacherous.
此句是对过去情况的假设：if从句谓语应用had done 形式；主句用would have done形式。所以从句应改成if the bridge had...