good - this word is an adjective that modifies a noun. in the sentence, "his salary is very good" it modifies the noun salary. well - is an adverb that modifies a verb,adjectives and another adverb. adverbs often answers the question "how" i can speak Filipino well. [how do i speak Filipino? - the answer is well.] in this sentence, the adverb well modifies the verb speak

good = adjective well = adverb

"Good" is an adjective, "well" is an adverb. In this sentence you need an adjective.