We'd better take his advice(改为否定句）答案上是 We'd better （not take） his advice我就想知道为什么回这样改,为什么不用don't 或者isn'tWe'd 是we would 的缩写吗

这是固定用法啊。。。
基本翻译
最好做某事
否定就是had better not do

因为 had better 已经是一种助动词，并且是习惯用法，连在一起的。所以不可以再加 dont 或 isnt

we'd better take his advice=(we)主语（ had better ）相当于情态动词take (his advice）宾语.
had better do sth的否定形式为 had better not do sth
原句的谓语动词前 有had better 所以要否定就只能否定had better而不是take
类似的句子有
we can take his advice >>>>we can not take his advice.
你自己说的
is 做谓语动词是要否定 is 所以是is not 即 isn't 例 he is a student>>he isn't a student
下面的句子是没有be 动词 和情态动词的情况
do 做谓语动词就要否定DO 所以是 do not 即 don't 例 he plays football well>>he doesn't play football well.

′d not
就是shouldn′t的简写，本身已经完整了。

had better do sth否定 直接加not
had better not do sth 固定语法