GMAT语法题，又称句子改错题(sentence correction)，是整个GMAT考试中对考生英语语法能力考查要求最高的题型，也往往是中国学生最为头疼的一类题目。没有扎实的语法基础和一定的逻辑分析能力，想要做好语法题难度极高。怎样做好GMAT语法题?下面小编就通过一些经典真题的实例解析，为大家全面讲解GMAT语法题的解题思路和方法技巧。
实例题目：
To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.
(A) have to know a great deal more than now about the social and economic
(B) have to know a great deal more than they do now about the social and economical
(C) would have to know a great deal more than they do now about the social and economical
(D) would have to know a great deal more than they do now about the social and economic
(E) would have to know a great deal more than now about the social and economic
选项分析：
(A)have to know a great deal more than now about the social and economic
【错误】 比较结构：本选项使用了比较句式，比较句式的本质是两个平行可比的主谓结构在进行比较，只不过其中之一会发生省略。“demographers have to know a great deal more than now”中“than”的后方连接的是一个省略后仅剩下“now”的主谓结构，由于“than”之前也是一般现在时，所以我们只能认为，“than”后方被省略的主谓结构还原后应完全继承前半句“demographers have to know”，即“demographers have to know a great deal more than(they have to know)now”，意思是“人口学家不得不知道(一般现在时)比他们现在不得不知道的更多”，表意明显不合理。相比之下，(E)中的“they do”则是“they know”
省略后的结果，表意为“人口学家将不得不(“would”是虚拟语气，表示实际无法实现)比他们现在知道的更多”。
(B)have to know a great deal more than they do now about the social and economical
【错误】 句子意思：“economical”意思是“节约的”而非“经济上的”。
(C)would have to know a great deal more than they do now about the social and economical
【错误】 句子意思：见(B)。
(D)would have to know a great deal more than they do now about the social and economic
【正确】 “would have to know a great deal more”表明“To develop more accurate population forecasts”当前条件下实际上是不可能的，虚拟语气使用合理。
【大意】 要进行更加准确的人口预测的话，人口学家将不得不比现在还要更多地了解生育的社会经济决定因素。
(E)would have to know a great deal more than now about the social and economic
【错误】 比较结构：类似地，见(A)。
解题思路：
“economic”和“economical”是同根形容词，他们一定不会表达同一个意思，而“economic”是“经济上的”的意思，那么“economical”一定有问题，排除(B)(C)。
当比较句式省略过度时往往会造成语义的不准确、不合理，，这正是(A)和(E)的情况，因此选择(D)。
