like 和as 是比较常见的比较词，like 是介词，也就意味着其后面需要加名词，代词或名词短语，不能加句子或介词短语。而对于as 可以当作介词或连词，也就是说其后面可以跟名词性的成分或句子：

　　◆ Wrong: LIKE her brother DID, Ava aced the test.

　　◆ Right: AS her brother DID, Ava aced the test.

　　上例中her brother DID 是一个句子，其为谓语动词是did，所以要用as来进行连接。

　　like 指相似的意思，作为介词后面必须跟名词，用来修饰前面的动词或名词，来形成一种比较。在SC部分，特别要注意在句尾的like :

　　(1) I want to coach divers LIKE Greg Louganis.

　　= I want to coach divers WHO ARE LIKE Greg Louganis.

　　OR

　　= I want to coach divers IN THE SAME WAY AS Greg Louganis does.

　　(2) I want to coach divers, LIKE Greg Louganis. 注意like 前面的逗号。

　　= LIKE Greg Louganis, I want to coach divers. (他coaches divers而我也想这么做)

　　unlike 在GMAT考试中很常见，其用法和like 相同，在句尾只要不产生歧义就行：

　　Most materials under a wide range of conditions resist the flow of electric current to some degree, UNLIKE superconductors, which demonstrate zero electrical resistance.

　　注：like 在平时会被人误用作表示列举，但在gmat中只有such as 才有这功能。

　　而as 可以作为介词也可以作为连词，当as 作为连词的时候通常的用法有这几种：

　　◆ 表示持续的As: AS I strolled to the store, I smelled the air. (= while, during)

　　◆ 表示原因的As: I will not tell you, AS you already know. (= since, because)

　　◆ 表示比较的As: You should walk AS she wants you to. (= in the same way)

　　而表示比较的as 在GMAT考试中是最重要的一种用法，有时会和just, so, 还有so too 连用。

　　◆ JUST AS the trains were late yesterday, the buses are late today.

　　◆ JUST AS the trains were late yesterday, SO TOO are they late today.

　　而作比较用的as 不单单只能边句子，也可以连接短语：

　　AS in the previous case, the judge took an early break.

　　当as 作为介词使用时，也基本满足下面三种用法：

　　◆ 表示作用的As: AS your leader, I am in charge. (= in the role of)

　　◆ 表示相等的As: I think of you AS my friend. (= you are my friend)

　　◆ 表示阶段的As: AS a child, I thought I could fly. (= when I was)

　　而对于当介词用的as 并没有相似的意思：

　　◆ I will jump up LIKE a clown. (= in a clownish manner)

　　◆ I will jump up AS a clown. (= in a clown suit!)

　　所以要使用as 来比较就要在句子中使用，最常用的as 的用法是as ... as... 第一个as 后跟形容词或副词，第二个as 后跟名词、短语甚至句子。