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.
One might note that the corresponding adjective "slow" does take the -ly suffix, but this has no impact on the behavior of "fast".
(There is also no real reason why, for example, we have warm/warmth, but cool/coolness.
Semantically related things sometimes have similar morphological patterns, and sometimes not.)
One interesting thing worth noting (that was brought up in a comment by Jimi Oke) is that there are cases of adjectives with identical adverbs that also have an -ly form; for example, we have "right" and "wrong" as adjectives and adverbs, but we also have "rightly" and "wrongly".
In such cases, the -ly form has carved out its own semantic niche; the adverb "right" and the adverb "rightly" cannot be used interchangeably in every situation.
I can say "turn right" and "rightly so", but I can't exchange them in either sentence.
With normal adjectives that cannot become adverbs without -ly , usually the -ly just transparently makes the adjective adverbial — it doesn't have its own separate semantic nuances (e.g. "quickly" simply means "in a quick manner").
english.stackexchange.com/users/449/Kosmonaut
The adverb form of fast is irregular.
It is one of several exceptions, as is "well" as mentioned by Dena A.
a fast runner / run fasta hard worker / work harda bad smell / smell bad etc.
etc.
etc.
About work hard: 'he works hard', in contrast to 'he is hardly working'.
Two different meanings.
english.stackexchange.com/users/None/
Fast is an adverb as well as an adjective, so you wouldn't use fastly .
Another common adverb that doesn't follow the pattern of ending in ly is well ,
not the expected goodly , which is actually an adjective and means a large quantity, e.g. a goodly sum .
english.stackexchange.com/users/697/Dena A
Per etymonline, fastly is the 'former adverbial form of fast (adj.), from O.E. fæstlic "firm, fixed, steadfast, resolute;" obsolete in 19c.
, simple fast taking its place.'
There you go folks, so much for logic...
english.stackexchange.com/users/None/
Though word fastly used in many places (you can find many results by searching Google ) but still it is not correct.
For reference see the definition of word fast from WikiDictionary See the post wrong / fast-- adverbs with no 'ly' ending to learn more.
english.stackexchange.com/users/40/chanchal1987
This reminds me of how small children extrapolate grammatical constructions in seemingly logical ways, except English is not always that logical.
So my daughter (3¾) will correctly say "I colour in neatly" or "I'll do it carefully", but then also "It comes lastly" "I can hop bigly" "Go farly" "Squeeze me hardly" etc.
english.stackexchange.com/users/1571/francois
Oh, fastly is a word alright.
It just happens to be an obsolete one.
english.stackexchange.com/users/12952/Barrie England
"Fast" can also mean to hold onto in a strongly, and "fastly" is the adverb of that word.
E.g. "The crew held fastly onto the hand-rail of the boat as it rolled in the storm"
english.stackexchange.com/users/708/Phillip Ngan
This is emblematic of a misunderstanding that gives rise to forms truly abominable, begetting debellished hypercorrections thusly seen in casual prose all over the internet todaily.
The problem is that people oftenly get this mistaken notion that adverbs need to end in ‑ly .
I know no likely explanation for this blunder, but they stilly do it anywaily.
Howeverly certain they are of this non-rule, nothing could be furthlier from the truth, and the soonlier they realize this, the morely their writing stands to gain; it sure won’t happen beforely.
This semisilly answer is intended to come across friendlily, not meanly or worsely.
english.stackexchange.com/users/2085/tchrist
Slow  has the adverb  slowly . I tend to use  fastly  as the adverb for  fast .  However, it is underlined in most spell checkers I use, which makes me wonder about the existence of this word.  Is  fastly  a correct word? If not, what should be used instead, and why is that different from its antonym?
Is "fastly" a correct word?
english.stackexchange.com
<adverbs><is-it-a-word>
english.stackexchange.com/questions/1413
english.stackexchange.com/users/370/Gnoupi
It is generally agreed that "fastly" is not necessary because "fast" is both an adjective and an adverb. However, it is possible that "fastly" as a word does exist but is simply obsolete. Th word "fast" is both an adjective and an (irregular) adverb and therefore there is no need for "fastly".
It is generally agreed that "fastly" is not necessary because "fast" is both an adjective and an adverb. However, it is possible that "fastly" as a word does exist but is simply obsolete. The adverb form of fast is irregular. Fast  is an adverb as well as an adjective, so you wouldn't use  fastly . There is the argument that "fast" can also mean to hold onto in a strongly, and "fastly" is the adverb of that word. Fastly  is used in many places (you can find many results by searching  Google ) but still it is not correct.
There is no need for "fastly" because "fast" is both an adjective and an adverb.   Fastly is not correct in this context because "fast" is both an adjective and an adverb. However, "fastly" might be acceptable if "fast" is being used in the sense of holding onto something tightly.
There is no need for "fastly" because "fast" is both an adjective and an adverb.   The adverb form of fast is irregular. It is a word, but it is obsolete.
The adverb form of fast is irregular. Fast  is an adverb as well as an adjective, so you wouldn't use  fastly . Fastly  is used in many places (you can find many results by searching  Google ) but still it is not correct. There is the argument that "fast" can also mean to hold onto in a strongly, and "fastly" is the adverb of that word.
"Fastly" is not correct in this context because "fast" is both an adjective and an adverb. "Fastly" is acceptable if used in the sense of "holding fastly" (strongly) to something.