GRAMMAR: GRAMMAR: well, goodGoodis an adjective• a good attempt • Her English is very good.!! Goodis not used as an adverb in standard English. Use well• He speaks English extremely well.The comparative form of both good and well is better• His first book was better.(adj)• We'll play better next time.(adv)The superlative form of both good and well is best• Who is the best singer?(adj)• You can use brown or white sugar, but brown works best.(adv)
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.