常见的助动词有be、have和do，对于助动词，常常可以帮助我们省去句子中一些比较长的部分：

◆ Wordy: I have never seen an aardvark, but my father has seen an aardvark.

◆ Better: I have never seen an aardvark, but my father HAS.

对于这种省略肯定需要时态上的一致，如果要转换时态就不能省略：

◆ Wrong: I have never seen an aardvark, but last year my father DID.

◆ Right: I have never seen an aardvark, but last year my father saw one.

我们再看一个例子：

◆ Wrong: Our cars were designed to inspire envy, and they ARE.

◆ Right: Our cars were designed to inspire envy, and they DO.

对于第一句，they are (inspiring )　因为时态的改变所以inspiring　不应该被省略，而第二句They DO inspire　没有什么问题。对于助动词而言，只能指代肯定的内容而不能指代否定句：

◆ Right: Some people do not eat soup, but others DO. (= do eat soup)

◆ Right: Some people do not eat soup as others DO. (= do eat soup)

在我们以前所学的过的英语中，有时会使用be to 来表示责任或义务或者是将来的一个时间，但在在GMAT中，这种表达方式容易让人产生误解，因而采用will 或should ：

◆ Wrong: We ARE TO receive an invitation.

◆ Right: We WILL receive an invitation. OR We SHOULD receive an invitation.

最后在GMAT考试中对于表示条件时应用if ，而不用should ：

◆ Awkward: SHOULD he PASS the test, he will graduate.

◆ Right: IF he PASSES the test, he will graduate