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"Fast" is both adjective and adverb; "quickly" is only an adverb and sounds more formal.
Side-by-side comparison
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
- •"Fastly" does not exist in English: "fast" is already an adverb on its own ("he runs fast").
- •The reverse error also occurs: "quick" does not work well as a formal adverb ("drive quick" is informal).
- •The correct adverb is "quickly" or "fast".
Trick to remember it
"Fast" = adjective OR adverb (same form). "Quickly" = adverb only, more formal. In conversation: "speak fast", "eat fast". In formal writing: "respond quickly". Never say "fastly": it does not exist.
Quick rule
"Fast" = adjective AND adverb (same form). "Quickly" = adverb only. Never "fastly".
Did you know?
"Fast asleep" perfectly demonstrates that "fast" can mean the opposite of speed. Here "fast" comes from Old English "faest" meaning "firm, fixed, immovable", same root as "hold fast" (grip firmly) and "stand fast" (remain firm). "Fast asleep" = so deeply asleep you are motionless. The "fast" meaning speed is a completely different semantic evolution of the same term. English is famous for such words called "contronyms" or "Janus words", words that are their own antonym.
Practise what you learned
Exercise 1 · Pick the right one
She ran _____ to catch the bus before it left.
Exercise 2 · Pick the right one
She _____ finished her dinner and headed out.
Exercise 3 · Form the word
Related word pairs
"Fast" describes continuous high speed; "quick" describes brevity or a short burst of speed.
"Hard" as an adverb means with effort or intensity; "hardly" means almost not, barely.
Both mean "not easy", but "hard" also means physically firm or working intensely.
'A' is used before consonant sounds; 'an' is used before vowel sounds.
Bryan López
Founder of SpeakUP Academy