SAT考试中的Grammar部分，一直以来是中国考生所擅长的部分，但仍存在部分知识盲点，下面就SAT的improving sentence（IS）部分和identifying sentence error（ISE）部分常出现的错误进行整理和归纳：
语法一：时态
众做周知，英语中含有十六大时态，但SAT中最常见的是一般现在时（因为与主谓一致常联系在一起，所以常考）、现在完成时（与句子的语义相关）、一般过去时以及过去完成时（与固定搭配常考）
SAT常考点：
1. 时态一致
一般来说，划线部分和未划线部分的时态相吻合，尤其是在主从复合结构和并列结构中。做题方法：关注未划线部分的时态，最好能在周围找到时间状语，推断出划线部分的时态。
例题：Dr. Chien-Shiung Wu has disproved a widely accepted theory of physics when she showed that identical nuclear particles do not always act alike.
A. has disproved
B. having disproved
C. disproved
D. disproves
E. disproving
题目解析：
划线部分和答案选项已经给予我们足够提示：时态问题。根据when后面的时态，我们得知从句的时态是一般过去时，根据时态一致，所以主句应该选择一般过去的C选项。但注意，在show后面出现的宾语从句中又出现了一般现在时，再根据句意：吴博士反驳了一个物理学界普遍认同的理论，她声称完全相同的原子核不总是作用相似（即真理）。所以后面是一般现在时。
2. 完成时态的标志词
大家在初高中必然学得使用现在完成时的条件，下面重新复习一下：
1）主句用现在完成时 since 一般过去式
例：Mike has prepared well for this trip since his father told that all the family would travel around western Europe during Christmas.
2）现在完成时 for 一段时间
例：I have trapped in this car accident for almost two hours.
3）现在完成时 in the past/last 一段时间
例：All the Americans have undergone griefs and restored their faith in life in the last decade.
4）现在完成时 表示现在完成时的副词
例： Recently, all the world have concerned about the Yellow Stone National Park because in <2012> the disaster starts there.
例题：Tenor George Shirley sang more than 20 leading roles at the Metropolitan Opera since having his debut there as Fernando in Cosifan tulle on October 24, 1961.
A. sang more than 20 leading roles at the Metropolitan Opera since having
B. sang more than 20 leading roles at the Metropolitan Opera since having had
C. has sung more than 20 leading roles at the Metropolitan Opera since
D. has sung more than 20 leading roles at the Metropolitan Opera after
E. has sung more than 20 leading roles at the Metropolitan Opera subsequently to
题目解析：根据未划线的部分，是一般过去时，然后又出现since，主句必然是现在完成时，所以选择选项C。
过去完成时也是SAT语法常考的知识点！
1）主句过去完成时 by the time/end 一般过去时
例：The flight had been away by the time Tom and his friends arrived at the airport.
2）主句过去完成时 before 一般过去时
例：All the class had already heard of this frustrating news before their teacher wanted to announce in class.
3）根据句意
例题：At holidays, my uncle was the relative who told me the stories that I had heard every year throughout my childhood and also the years of my adolescence.
A. had heard every year throughout my childhood and also the years of my adolescence
B. had heard every year throughout my childhood and my adolescence
C. have heard repeatedly throughout my childhood and my adolescence alike.
D. have, every year, had told to me both in my childhood as well as my adolescence.
E. every year have had told to me throughout my childhood and my adolescence.
题目解析：未划线部分为一般过去时，划线部分理解为：我已经在青少年时期每年都听过的故事，这就比主句动作的提前一个时间段发生，所以应为过去完成时。又根据SAT语法中讲究省略原则，所以当出现and also the years of这样的结构，是重复了后面的my adolescence，所以选择B选项更为简洁。
总之，过去完成时的出现要以先出现过去时或者过去进行时为前提，然后根据before/ after或者句意来判断是否该使用过去完成时。
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