大多数同学在做题中往往不能有效的快速判断一个题的全部考点，或者考点找偏，这样都会大大降低我们做题的正确率和pace。
今天我们就来说一说这个经常会被同学们忽略的语法点-介词以及介词短语的用法，结合题目为大家介绍几个常见的介词知识点。
As
当这个词为介词词性时，只能是“作为”的意思。比如As a teacher, as a student….
所以GMAT语法中往往会和like在比较结构的题目中一起出现在备选项中，那么我们要知道，当表示“像”的含义时，as只能是连词。
例题1
As with those of humans, the DNA of grape plants contains sites where certain unique sequences of nucleotides are repeated over and over.
A. As with those of humans, the DNA of grape plants contains sites where
B. As human DNA, the DNA of grape plants contain sites in which
C. As it is with human DNA, the DNA of grape plants, containing sites in which
D. Like humans, the DNA of grape plants contain sites where
E. Like human DNA, the DNA of grape plants contains sites in which
In
in表示“在什么里面”或“在什么方面”。例如：There are so many apples in the bag./He is proficient in speaking English.

例题2
According to United States Air Force officials, a cannon shooting dead chickens at airplanes has proved helpful to demonstrate what kind of damage can result when jets fly into a flock of large birds.
A. shooting dead chickens at airplanes has proved helpful to demonstrate
B. shooting dead chickens at airplanes has proved itself helpful as a demonstration of
C. shooting dead chickens at airplanes proves itself helpful as demonstrating
D. that shoots dead chickens at airplanes proves itself helpful to demonstrate
E. that shoots dead chickens at airplanes has proved helpful in demonstrating
To
to接名词表示对象，to do 表示目的，不存在就近修饰的用法。经常和“for doing…”在选项中一起出现,一般情况下我们都会选择to do 不定式，当然也不是**的，要结合语义判断表示原因还是表示这个动作的目的。

例题3
Most states impose limitations on the authority of the legislature to borrow money, with their objectives being to protect taxpayers and the credit of the state government.
A. to borrow money, with their objectives being to protect
B. to borrow money, the objectives of which are the protecting of
C. to borrow money, limitations intended to protect
D. for borrowing money, of which the objective is protecting
E. for borrowing money, limitations with the intent of protecting
By
by表示”通过什么方式”，等同于through的用法，往往也会在选项中和介词短语in一起考察。
例题4
Each year companies in the United States could save as much as $58 billion annually by preventing illness among employees and gain as much as $200 billion through improving performance of workers if they simply provided offices with cleaner air.
A. annually by preventing illness among employees and gain as much as $200 billion through improving performance of workers if they simply provided
B. annually if they prevented employee illness and gain as much as $200 billion through worker performance improved by simply providing

C. annually in employee illness prevention and gain as much as $200 billion through worker performance improved by simply providing
D. in employee illness prevention and gain as much as $200 billion through improving performance of workers if they simply provided
E. by preventing illness among employees and gain as much as $200 billion through improved worker performance if they simply provided
On
on表示”在什么方面”， 一般考察的比较少，通常涉及到一些固定用法。比如：focus onconcentrate on 等。
除了掌握这些介词的含义，同学们还需要了解和介词有关的一些语法规律。
介词放句首三原则介词短语表示方式，时间，地点，作状语，修饰句子，不要求逻辑主语一致。介词短语表示比较：如besides, like等，要求逻辑主语一致。介词加上一个动作构成短语，要求逻辑主语一致，因为动作的执行者是主语。