Would it be valid to change the いい in (verb)ても良い into a negative?


E.G. 言わなくても良くない

I've always been curious about this, but none of the books or classes I've had
ever approached it




～てもいい basically means "It's okay even if you do...", eg:

> 「言っ **てもいい** 」 -- "It's okay even if you say" → "It's okay to say / You
> _may_ say"  
>  「言わ **なくてもいい** 」 -- "It's okay even if you don't say" → "You _don't have
> to_ say"

To say "You may not do..." or "You have to do...", you'd use ～（なく）て **は**
いけない, rather than ～（なく）て **も** 良くない (×も→◎は):

> 「言っ **てはいけない** 」 -- "It won't do if you say" → "You _may not_ say"  
>  「言わ **なくてはいけない** 」 -- "It won't do if you don't say" → "You _have to_ say"

～て **も** 良くない, ～て **も** いけない would usually mean "Doing ... is not good
(either)":

> 「食べすぎても良くない/いけないが、全く食べなくても良くない/いけない。」  
>  or 「食べすぎるのも良くない/いけないが、全く食べないのも良くない/いけない。」  
>  "Eating too much is not good, but not eating at all is not good, either."

You'd also use ～なくても良くない？ at the end of a sentence in casual conversation,
like this:

> 「学校なんて行かなくてもよくない？」 -- "You don't need to go to school, right?"  
>  「そんなこと言わなくてもよくない？」 -- "You don't need to say that, do you?" "Do you really
> need to say that?"




～てもいい basically means "It's okay even if you do...", eg:

> 「言っ **てもいい** 」 -- "It's okay even if you say" → "It's okay to say / You
> _may_ say"  
>  「言わ **なくてもいい** 」 -- "It's okay even if you don't say" → "You _don't have
> to_ say"

To say "You may not do..." or "You have to do...", you'd use ～（なく）て **は**
いけない, rather than ～（なく）て **も** 良くない (×も→◎は):

> 「言っ **てはいけない** 」 -- "It won't do if you say" → "You _may not_ say"  
>  「言わ **なくてはいけない** 」 -- "It won't do if you don't say" → "You _have to_ say"

～て **も** 良くない, ～て **も** いけない would usually mean "Doing ... is not good
(either)":

> 「食べすぎても良くない/いけないが、全く食べなくても良くない/いけない。」  
>  or 「食べすぎるのも良くない/いけないが、全く食べないのも良くない/いけない。」  
>  "Eating too much is not good, but not eating at all is not good, either."

You'd also use ～なくても良くない？ at the end of a sentence in casual conversation,
like this:

> 「学校なんて行かなくてもよくない？」 -- "You don't need to go to school, right?"  
>  「そんなこと言わなくてもよくない？」 -- "You don't need to say that, do you?" "Do you really
> need to say that?"

