急i would have been there on the spot had it not been for the

这是虚拟语气,并且是对过去的虚拟,表示对过去的假设与过去的事实不符.
本来应该是：I would have been there on the spot if it had not been for the traffic problem.
在这种假设虚拟语气中,并且有had、shoud等情态动词时,可以省略if,而把had或should提前.
如果不要if仅仅是 it had not been for 肯定是不行的,这样的话句子里有两个谓语了!

此题考的是虚拟条件句的倒装。
这个句子可还原为：I would have been there on the spot if it had not been for the traffic problem.
在if虚拟条件句中，若有had,were,should时，可省if，将这些词提到条件句的句首，采用倒装形式。