If I knew how to explain I'd have written the explanations with the original post I guess "never have I seen" can also be correct , doesn't sound unnatural... but "so good sentence" is definitely wrong... better "such a good sentence" ... if you use "so good" then the form is "so good of a [what]"...

The way you said shows us exactly using adjectives instead of adverbs.
Yea I meant that you use adverbs when you need adjectives