The difference between “fast”, “fastly” and “quickly” in English

A common mistake among English learners is using “fastly” to mean “quickly”, which seems logical because adverbs are commonly formed in English by adding -ly to the end of an adjective. For example, if something is slow, we can say that it moves slowly: it moves slowly.

Unfortunately, languages don’t always develop in a completely logical way, and the word “fast” is an exception to the above rule: “fast” and “fast” are both said “fast”, and the word “fastly” doesn’t exist in English at all. So we would say, for example, “fast” and “fast” are not even English words.

  • The athlete runs really fast. (correct)
  • The athlete runs really fastly. (incorrect)

The word quickly is an adverb formed from the adjective quick and means basically the same thing as fast, except that it can be used to mean that something has to happen in a short time. For example, if we say “quick”, we can say “fast”.

  • Come here, quickly!

it’s important to us that the person comes soon, but we probably don’t care how fast they move. However, it is possible to use quickly even when we are talking purely about movement, as in “he runs quickly”.

There is one situation, however, where it is not possible to use fast and it is necessary to use quickly or another adverb ending in -ly – if the adverb comes before the verb to which it refers:

  • He quickly ran out of the building. (correct)
  • He fast ran out of the building. (incorrect)
  • He fastly ran out of the building. (incorrect, “fastly” does not exist)

The difference between “fast”, “fastly” and “quickly” in English

The difference between fast and quick in English is that the former is an adjective, while the latter is a mere adverb. As such, the adjective is a double duty. However, it doesn’t need the -ly suffix.

In the context of adverbs, the word -ly isn’t strictly necessary, but it does make the adjective a bit more apt. This isn’t to say that -ly isn’t used in English. On the contrary, the -ly suffix isn’t always applied to normal adjectives.

For instance, the Oxford English Dictionary lists “fast” as an adverb before 1275, but not before. That is not to say that the word is not a useful adjective in modern usage. Its standard synonyms are more readily available than those for slow.

Likewise, the word “quick” isn’t used as often as its adverbial cousin, but it isn’t impossible to find a quick substitute. A quick search on Google tells me that the average person has at least one, if not many, “quick” adverbs in their vocabulary.

If you’re looking for the best way to use the word fast, try replacing it with a more appropriate adjective. You can even make it a verb. Just don’t confuse it with the adjective.

While the differences between the words are slight, the adverb is the real show off. Fortunately, a quick search on Google will reveal that the adverb is a bit pricier than its adjective counterpart. But that doesn’t mean that you shouldn’t be a bit careful with it.