fast に -ly が付かない理由(同形の副詞)Why There's No "Fastly": Flat Adverbs
形容詞・副詞 / Adjectives & Adverbs
fast は形容詞(速い)も副詞(速く)も同じ形で、fastly という単語は存在しません。early・late・daily・straight も同じ「同形組」。「副詞= -ly」の公式に乗らないメンバーを、ここで名簿にしておきます。
Fast is both the adjective (a fast car) and the adverb (drive fast) — "fastly" doesn't exist. Early, late, daily, and straight belong to the same shape-sharing club. Here's the member list for everyone the "-ly = adverb" formula misses.
The main members: fast, early, late, hard, high, low, straight, and the daily / weekly / monthly set — each one wearing the same face as adjective and adverb. Fast, early, and late are the highest-frequency members, which means the urge to bolt on a -ly strikes constantly.
同じ顔で二役One face, two jobs
He's aなぜ a なのか【職業・種類・性質を言う】He's a cook. のように「〜という種類の人・もの」だと述べる形。日本語の「〜だ」に当たる部分で、英語は冠詞を落とせない。 fast runner. He runs fast.
彼は足が速い。速く走る。
形容詞も副詞も fast。fastly は存在しない。Fast covers both — "fastly" isn't a word.
I caught anなぜ an なのか【初めて出す一つ】聞き手がまだ知らないものを初めて話に出すから a。次に同じものを指すときは the に変わる。 early train and got up early.
早い電車に乗った。早起きした。
early も同形。Early shares its shape too.
Theなぜ the なのか【その場で一つに決まる】話し手と聞き手のいる状況で、どれを指すか一つに決まるから the。日本語なら「（例の）」「うちの」を補って読むと感覚が近い。 bakery is open daily.
そのパン屋は毎日開いている。
daily・weekly・monthly も一人二役(a daily paper / published daily)。Daily, weekly, monthly double up as well.
同形組(fast・early・late)
-ly なしでそのまま副詞に。Adverbs with no -ly at all.
Don't talk so fast.
そんなに速くしゃべらないで。
通常組(quick → quickly)
ふつうの形容詞は -ly で副詞化する。Ordinary adjectives still take the -ly route.
She answered quickly.
彼女はすばやく答えた。
よくある間違い / Common Mistake
×Please reply as fastly as possible.
できるだけ早くご返信ください。(のつもりで)
○Please reply as fast as possible.
できるだけ早くご返信ください。
fastly という単語は現代英語に存在しません(存在するのは CDN のサービス名くらいです)。fast はそのままで副詞。ビジネスなら as soon as possible(ASAP)がさらに定番です。
"Fastly" isn't a modern English word (outside a certain CDN's brand name). Fast is its own adverb. In business writing, as soon as possible — ASAP — is the even more idiomatic choice.
as fast as possibleできるだけ速く同形副詞の実戦形。as ... as の枠にそのまま入る。The flat adverb in its working clothes — straight into the as … as frame.
sooner or later遅かれ早かれsoon(すぐに)も -ly を取らない古参の副詞。比較級で慣用に。Soon is another veteran that never took -ly — here fossilized in comparison.
go straightまっすぐ進む道案内の必須表現。straight は同形組の代表格。Essential for directions — straight, the club's poster member.
work late残業する(遅くまで働く)late の副詞用法の定番。lately(最近)と混ぜないこと。Late the adverb at its busiest — never to be confused with lately.
まとめ / Summary
fast・early・late・straight・daily は形容詞=副詞の同形。Fast, early, late, straight, daily: adjective and adverb in one shape.
hard/hardly 型(意味が変わる組)とは別名簿(「hardly・lately・nearly」の項を参照)。The meaning-switching pairs (hard/hardly) are a different roster — see the hardly, lately, nearly entry.