TYPICAL ERROR The engine was revving very loud.
CORRECTION The engine was revving very loudly.
EXPLANATION Adjectives are used to qualify nouns. For example, we might talk about a loud noise. Adverbs, on the other hand, modify verbs, adjectives and other adverbs. Thus, we should say that the engine was revving... loudly (because an adverb is required to describe the verb was revving). Notice that the adverb loudly ends in -ly. Most adverbs end this way (or -ally after the letter c, as in tragically).
Using adjectives instead of adverbs is generally just a habit that some people develop if they hear others doing the same thing.
Make adverbs from these adjectives by adding -ly or -ally.