之前学习了形容词比较等级的构成方式，而形容词比较等级用于句子当中便形成了所谓的“比较结构”。英语比较结构最主要的有三种：as。。。as结构，more。。。than结构以及the most。。。结构。as。。。as结构意为“和。。。一样”，其用法主要如下：
一、基本模式：as+形容词原级+as-分句
1、主句主语与as-分句主语不同，比较项目相同，此时as-分句有所省略。如：
Jack is as clever as Tom。
2、主句主语与as-分句主语相同，但比较项目不同，此时as-分句不可用省略句。如：
The girl is as brilliant as she is beautiful。 （这个女孩既漂亮又聪明。）
3、主句主语与as-分句主语不同，比较项目也不同，此时as-分句不可用省略句。如：
Her uncle was as base and unworthy as her father had been upright and honorable。
（她叔叔卑鄙龌龊，不像她父亲那样正直可敬。）
4、基本模式的否定形式：
Jack is not as clever as Tom。 或 Jack is less clever than Tom。
二、变体模式：
1、as many/much+名词+as-分句：
She has eaten as many apples as her sister。
He took as much water as he needed。
2、as+形容词原级+名词词组+as-分句：
Lucy is as clever a girl as her sister。
3、变体模式的否定形式：
He didn’t take as much water as he needed。
需要注意的是，同一句话，采取基本模式和变体模式，它们的含义基本相同，但句子的侧重点有所不同。例如：
基本模式：I haven’t seen a car as old as this for years。（重点在名词car）
变体模式：I haven’t seen as old a car as this for years。（重点在形容词old）
三、as。。。as结构常用于形象比喻中：
as rich as a jew 极其富有
as cool as cucumber 沉着冷静
as light as a feather 轻如鸿毛
as soft as butter 性格软弱
as poor as a church mouse 一贫如洗
as clear as crystal 清澈透明，一清二楚
as busy as a bee 忙忙碌碌
as easy as ABC 相当容易
as deep as a well 高深莫测
as stubborn as a mule 非常顽固，倔脾气