Re: "She is a good singer" or "She sings well"
Loud IS an adverb, but not as it applies to volume of sound -- only in offensiveness, like "a loud pink dress."
But what does misusing "loud" as an adverb have to do with "good" being incorrect when you use it to say she's a good SINGER?
She sings good? That's clearly wrong.
She sings well. Perfectly fine.
She's a good singer. Perfectly fine and to me, something I'd be more likely to say.
Did you hear Terri at karaoke? Who knew she was such a good singer?! (
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.