There is no need for "fastly" because "fast" is both an adjective and an adverb. So, "I ran fast" is completely correct. The existence of "fast" as an adverb does not preclude the future development of a word "fastly", but it does hinder it.
0
Pranav Kumar answered this
the word fast is adjective as well as a adverb
so there is no adverb for fast
