GMAT考试中被动与主动是常出考点的地方。在被动式中的发起者常用介词by 来引导出，而如果是方式或工具则用through 或because of。对于这种表达方式如果使用主动则会显得句子过长并且容易造成歧义：

◆ Wrong: The pizza WAS accidentally EATEN BY a quirk of fate.

◆ Wrong: A quirk of fate accidentally ate the pizza.

◆ Right: THROUGH a quirk of fate, the pizza WAS accidentally EATEN.


但是被动的句子不都是错误的句子，只是在意思相同的情况下有时会稍逊于主动表达的句子：

◆ Passive: It HAS BEEN DECIDED by Jason that he will not attend college.

◆ Active: Jason HAS DECIDED not to attend college.


但是这并不代表所有被动都不正确，只是说在句意相同的情况下主动更好。我们再看一个例子：

During this operation, new blood vessels are inserted, bypassing blockages.

上面的这句话使用了inserted 这个词，并没有产生任何歧义或者冗余，上面的话强调了blood vessels inserted而非谁发起了这个动作。对于被动态出现的标志词是by ，而在句子中并不一定非要被动和主动出现平行，当然如果能平行就更好了。