Adjectives and adverbs are used to perform different functions in phrases and sentences. Adjectives are used to modify nouns and to ascribe properties to the subject or object of a clause, whereas adverbs are used to modify adjectives and adverbs. Adverbs also function as adverbials at the clause level.
It is common in informal spoken English to use adjectives in adverbial functions. However, in formal writing this usage is not appropriate. Thus, the use of adjectives in examples like the following are characteristic of informal speech and would be regarded as errors in a formal written context (and also in careful speech):
(1) You must be real careful not to touch the wire. [In academic writing, the adverb really would be used to modify the adjective careful]
(2) Bill performed the task good. [In academic writing the adverb well would be used to fulfil the role of manner adverbial]