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。