“Fast” / “fastly” / “quickly” in English

by Jakub Marian

Tip: See my list of the Most Common Mistakes in English. It will teach you how to avoid mis­takes with com­mas, pre­pos­i­tions, ir­reg­u­lar verbs, and much more.

Many English learners use the word “fastly” as an adverbial form of “fast”, which seems quite logical because this is the way adverbs are usually formed. If something is slow, we say that it “moves slowly”, and such usage is completely correct. Nevertheless, languages develop in a way that is not always logical, and the situation of “fast” vs. “fastly” falls exactly into this category.

The fact is that there is no such word as “fastly”. “Fast” is already both an adjective and an adverb, as in

correct The athlete runs really fast.
wrong The athlete runs really fastly.

It doesn’t matter whether “fast” refers to the speed of movement or the rate at which something is being done; it’s always just “fast”:

correct He can’t write fast enough.
wrong He can’t write fastly enough.

The word “quickly”, used as an adverb, is synonymous with “fast”, but usually refers to the time an action takes rather than to the speed of movement. For example, you can say

Come here, quickly!

which means that you want the other person to come soon; you don’t really care how fast he or she is moving. Nevertheless, “quickly” can also refer to the actual speed (as in “he runs quickly”), but such usage is less common.

There is one situation in which it is obligatory to use “quickly” (or “swiftly” or a related adverb ending with “-ly”)if an adverb precedes the verb it modifies:

correct He quickly ran out of the building.
wrong He fast ran out of the building.
wrong He fastly ran out of the building.

This article was based on my guide to the most common mistakes in English, which explains many similar topics. Why don’t you check it out?