Some flat adverbs sound perfectly natural to most speakers, like “play nice” or “drive safe”. Others have less acceptability among people in general, like “That tastes real good.”
There’s that old line that if my aunt had wheels she’d be a bicycle. Maybe the command form is muddling the topic here, but using the be-verb with an adjective like that attaches a subject complement, essentially describing the subject. But “I am fast” describing a person doesn’t mean that saying “I drive fast” is describing a drive as a noun.
Some flat adverbs sound perfectly natural to most speakers, like “play nice” or “drive safe”. Others have less acceptability among people in general, like “That tastes real good.”
https://en.wikipedia.org/wiki/Flat_adverb
Tastes real goodly.
I don’t even see “nice” in “play nice” as an adverb. You could switch “play” for “be” – “be nice”, same with “be safe”.
Playly not nice.
There’s that old line that if my aunt had wheels she’d be a bicycle. Maybe the command form is muddling the topic here, but using the be-verb with an adjective like that attaches a subject complement, essentially describing the subject. But “I am fast” describing a person doesn’t mean that saying “I drive fast” is describing a drive as a noun.