The word 'fast' can be either an adjective or an adverb. Click here to read its definitions.
Student or Learner
If that "beautiful" in "She's so beautiful" is a adjective is right, then, why "fast" is a adverb in "don't drive so fast"?
In the second example, the sentence's verb is not a linking verb. The word "fast" modifies the verb "drive"; it is, therefore, an adverb in that use.
His driving is so fast -> fast is an adj. He drives so fast -> fast is an adv. That is correct ?
It make sense!