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I am not by any means a poetry expert, but I know a bit about grammar and writing. Ergo, I can say that in Dylan Thomas’s most famous poem, Do not go gentle into that good night, the refrain that the poem takes its name from is ungrammatical. The grammatically correct way to write (or say) this would be “Do not go gently into that good night.” What I want to know is as follows:
a) Is there a historical usage of “gentle” as an adverb and b) if there isn’t, why did Thomas determine that it was okay to use an adjective adverbially? Or is there some poetry-grammar exception that I am unaware of?