"not as...as"是英语中同级比较的否定形式，用来表示两者在某方面“不相等”或“前者不如后者”，核心意思是：
"not as...as"的结构需遵循“原级比较”的规则（中间必须用形容词/副词的原级，不能用比较级或最高级），具体分三种情况：
结构：主语 + be动词 + not as + 形容词原级 + as + 比较对象
例：
She is not as tall as her mother.（她不如她妈妈高。）
This movie is not as interesting as that one.（这部电影不如那部有趣。）
结构：主语 + 助动词（do/does/did）/情态动词 + not + 动词 + as + 副词原级 + as + 比较对象
例：
He doesn't run as fast as Tom.（他跑得不如汤姆快。）
I can't speak French as well as you.（我法语说得不如你好。）
若比较的是“数量多少”，需用not as many...as（修饰可数名词复数）或not as much...as（修饰不可数名词）：
结构：主语 + 动词 + not as many/much + 名词 + as + 比较对象
例：
There aren't as many students in our class as in theirs.（我们班的学生不如他们班多。）
I don't have as much money as my brother.（我没有我哥哥那么多钱。）
1、 必须用“原级”：中间的形容词/副词不能变比较级（如不能说"not as taller as"，需用"not as tall as"）。
2、 比较对象要明确：比较的对象可以是名词、代词，或省略重复部分（如"She doesn't sing as well as before."她唱歌不如以前好）。
3、 与"not so...as"的互换：在大部分情况下，"not as...as"和"not so...as"可以通用（"so"更正式，"as"更口语），如：
He is not so clever as his sister. = He is not as clever as his sister.（他不如他姐姐聪明。）
形容词：My bag is not as heavy as yours.（我的包不如你的重。）
副词：She didn't study as hard as her friend last term.（上学期她学习不如她朋友努力。）
数量：We didn't eat as much food as they did.（我们吃的食物不如他们多。）
简单来说，"not as...as"就是用“原级”把两个事物的某方面“比下去”，重点在“前者不如后者”~