If I understand your question, then this is my answer. "Slow" is an adjective, so it modifies a noun: She is a slow reader. "Slowly" is an adverb, so it modifies a verb: He is walking slowly. I admit though that many people, even native speakers, do this incorrectly.

Yes, there is a tendency to use adjective(slow) where adverb(slowly) is called for. * Now you count it again, real slow. (Should be "really slowly" to be grammatically correct) * I want it bad. (Should be "I want it badly") I think most of the time it's not that they don't know it, but the "slow" or "bad" sounds so much cleaner and more compelling than "slowly" or "badly". So they say it for the effect.

People have problems with adjectives and adverbs... Almost everyone confuses 'fast and quickly'... For example: This is a fast car. The car drives quickly. / He is a fast talker. He talks quickly.