"Fast" is both adjective and adverb; "quickly" is only an adverb and sounds more formal.
Quick, rapid. As an adjective it describes something (a fast car). As an adverb it describes how something is done (run fast). It does not change form.
She is a fast runner. She finished the race in under four minutes.
Time passes so fast when you are having fun.
Don't eat so fast. You'll get a stomachache.
Rapidly. Only an adverb. More formal and natural in writing; less common in everyday speech.
She quickly realized that something was wrong with the plan.
He packed his bags quickly and left without saying goodbye.
Please respond quickly. The deadline is in one hour.
Common mistake
The most common error: saying "fastly" — that word does not exist in English. "Fast" is already an adverb on its own: "he runs fast" (correct), "he runs fastly" (incorrect). Another frequent error: confusing "quick" and "quickly" — "quick" is an adjective (a quick response), "quickly" is an adverb (respond quickly). "Fast" can do the work of both in many contexts.
Trick to remember it
Fast = adjective or adverb (same form). Quickly = adverb only, more formal. In conversation: "speak fast", "drive fast", "eat fast". In formal writing or when you want to sound more polished: "respond quickly", "he quickly understood". Never say "fastly" — it does not exist. If in doubt between "fast" and "quickly" as an adverb, both work in most contexts.
Quick rule
Fast = adjective AND adverb (same form). Quickly = adverb only. Never "fastly". In conversation, "fast" as an adverb is the most natural choice.
Did you know?
"Fast" as an adjective has a completely opposite meaning in an old but still-used expression: "fast asleep" = deeply asleep (not quickly asleep). Here "fast" comes from Old English "faest" meaning "firm, fixed, immovable". So "fast asleep" = solidly/firmly asleep. The same origin gives us "hold fast" (grip firmly) and "stand fast" (remain firm). The "fast" meaning speed is a more recent semantic evolution.
Exercise 1 · Pick the right one
She _____ understood what the professor was explaining and took detailed notes.
Exercise 2 · Pick the right one
That is a really _____ car. What engine does it have?
Exercise 3 · Form the word
Fast = adjective AND adverb (same form). Quickly = adverb only. Never "fastly". In conversation, "fast" as an adverb is the most natural choice.
The most common error: saying "fastly" — that word does not exist in English. "Fast" is already an adverb on its own: "he runs fast" (correct), "he runs fastly" (incorrect). Another frequent error: confusing "quick" and "quickly" — "quick" is an adjective (a quick response), "quickly" is an adverb (respond quickly). "Fast" can do the work of both in many contexts.
Fast = adjective or adverb (same form). Quickly = adverb only, more formal. In conversation: "speak fast", "drive fast", "eat fast". In formal writing or when you want to sound more polished: "respond quickly", "he quickly understood". Never say "fastly" — it does not exist. If in doubt between "fast" and "quickly" as an adverb, both work in most contexts.
Bryan López
Founder of SpeakUP Academy