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：
The Organization of Petroleum Exporting Countries (OPEC) had long been expected to announce a reduction in output to bolster sagging oil prices, but officials of the organization just recently announced that the group will pare daily production by 1.5 million barrels by the beginning of next year, but only if non-OPEC nations, including Norway, Mexico, and Russia, were to trim output by a total of 500,000 barrels a day.
(A) year, but only if non-OPEC nations, including Norway, Mexico, and Russia, were to trim output
(B) year, but only if the output of non-OPEC nations, which includes Norway, Mexico, and Russia, is trimmed
(C) year only if the output of non-OPEC nations, including Norway, Mexico, and Russia, would be trimmed
(D) year only if non-OPEC nations, which includes Norway, Mexico, and Russia, were trimming output
(E) year only if non-OPEC nations, including Norway, Mexico, and Russia, trim output
选项分析：
(A)year, but only if non-OPEC nations, including Norway, Mexico, and Russia, were to trim output
【错误】 平行结构：“but”后方是“if”引导的从句，“but”前方是主句，主句和从句语法功能不一致，无法用平行结构词“but”连接。
(B)year, but only if the output of non-OPEC nations, which includes Norway, Mexico, and Russia, is trimmed
【错误】 平行结构：见(A)。
(C)year only if the output of non-OPEC nations, including Norway, Mexico, and Russia, would be trimmed
【错误】 动词时态：过去将来时的“would be trimmed”是“if”引导的从句的谓语动词，其主句是“the group will pare daily production by 1.5 million barrels by the beginning of next year”，是一般将来时，时态差异须有合理解释，本选项的时态差异不存在合理解释，表意逊于(E)中的“trim output”——主将从现。
(D)year only if non-OPEC nations, which includes Norway, Mexico, and Russia, were trimming output
【错误】 动词时态：类似地，见(C)。
【错误】 主谓一致：复数的“non-OPEC nations”与“which includes”主谓不一致。
(E)year only if non-OPEC nations, including Norway, Mexico, and Russia, trim output
【正确】 一般现在时的“trim output”是“if”引导的条件状语从句的谓语动词，与其一般将来时的主句“the group will pare daily production by 1.5 million barrels by the beginning of next year”的时态差异可以用“主将从现”来解释。
【大意】 OPEC长期以来被期望会减产而提振油价，但该组织官员近期宣布，只有包括挪威、墨西哥和俄罗斯等非OPEC国家合计每日减产50万桶以上，OPEC组织才将会从明年开始将日产量减少150万桶。
解题思路：
“but”两端应连接平行结构，立即排除(A)、(B)。对比(C)、(D)、(E)，“if”引导的条件从句和其主句的时态差异并不相同。
仅有(E)的时态差异存在“主将从现”的合理解释，因此选择(E)。
