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)