形容词和副词比较等级用于句子当中便形成比较结构。英语比较结构最主要的有三种：
as.....as 结构， more ...than 结构和 the most ...结构。英语中有几个不定限定词和不定代词也有原级、比较级和最高级。它们是 many/much ,more , most ;few , fewer , fewest;little ,less ,least。
1： as ......as 结构
这种结构的基本模式是： as +形容词或副词原级 +as -分句。
eg:
John is as bright as Bob.
John behaves as politely as Bob (does ).
This parcel is as heavy as that one.
This car runs as fast as that one.
这种比较结构的否定形式是：
John is not as /so bright as Bob.
John doesn't behave as /so politely as Bob.
以上否定意义还可用less....than结构来表示：
John is less bright than Bob.
John behaves less politely than Bob.
在带有as.....as结构的句子中，通常是主句主语和as-分句的主语不同，而比较项目相同，这是as....as结构最基本的用法。比外，as.....as结构的句子也可能主语相同，而比较项目不同。
eg:
The girl was as brilliant as she was beautiful.（这姑娘既聪明又漂亮）
This swimming pool is as wide as it is long .（这个游泳池宽度和长度相同）
He is not so wise as he is witty .（他为人风趣，但有欠明智）
She is not so witty as she is pretty .（她相貌美丽，但不太风趣）
也可能两个主语和两个比较项目都不同，即两个不同的人或物就不同的方面进行比较。
eg:
This swimming pool isn't twice as wide as that one is big .（这个游泳池额宽度并没有那个游泳池的两倍 。）
当主句主语和as-分句主语不同而比较项目相同时，as-分句总是有所省略，即通常用不完全句。但若两个主语相同而比较项目不同或者两个主语和两个比较项目都不相同，as-分句便不能用省略句，而必须用完全句。
as.....as结构的另一种模式是：as much/many +名词+ as-分句，这时many/much 是限定词。
eg:
He took as much butter as he needed.
She has written as many essays as her brother (has).
这种结构的否定形式是：
He didn't take so/as much butter as he needed.
She hasn't written so/ as many essays as her brother (has).
as......as 结构还有一个变体形式，这就是as +形容词原级+名词词组+as-分句。
eg:
George is as efficient a worker as Jack.
I don't want as expensive a car as this .
Hardly ever have i heard as impressive music as this .
I can't drink as sweet coffee as this .