いいじゃない vs よくない, whats the difference?


So in genki 1, it said that いい is changed to よ when being negative and it
becomes よくない, and not いいじゃない. But I was watching a show and someone said
いいじゃないですか. So now I'm kind of confused, can someone explain this to me?

Thank you




[いい](https://dictionary.goo.ne.jp/word/%E3%81%84%E3%81%84/#jn-9174) is a
colloquial form of the adjective よい and is used only in dictionary/prenoun
forms. Adjectives are negated not by じゃない, but by conjugation: よい→よくない,
青い→青くない.

[じゃない](https://jisho.org/search/%E3%81%98%E3%82%83%E3%81%AA%E3%81%84#) in
いいじゃない means _isn't it_.

To clarify:

  * Xはよい=Xはいい：X is good
  * Xはよくない：X isn't good
  * Xはいいじゃない：X is good, isn't it? (よいじゃない as a combination should be less common, because よい is less colloquial and じゃない is colloquial)

