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.
= 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.
AS in the previous case, the judge took an early break.
◆ 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)
◆ 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。
Never before had taxpayers confronted so many changes at once as they had in the Tax Reform Act of 1986.
A.so many changes at once as they had in
B.at once as many changes as
C.at once as many changes that there were with
D.as many changes at once as they confronted in
E.so many changes at once that confronted them in
此题意思应该都能看懂 说的是从来没遇到过这么多的事与１９８６年的时候相比 看选项
Ｃ there were with？这像人类表达吗
Ｅconfronted them in也是个不着边的选项