Why is "Let's do it quickly" correct but not "Let's do it fastly/soonly"?

The adverbial form of *fast* is *fast*, not *fastly*; English has a number of short, very common adjectives like that. And *soon* is always an adverb. So you would, and can, say: "Let's do it fast"; "Let's do it soon."

Honestly? Because fastly/soonly aren't words. You would be understood if you said it, but people would look at you a little weird. There's probably a better, more in depth answer, but it's just not a word in modern English. It's often either "quickly" or "fast-paced"

Short answer: because fastly and soonly are not words that exist Soon is already an adverb, so there's no need to add the -ly adverb ending. Fast is one example of an adjective that does not get the -ly ending when it is changed into an adverb. This is becoming more common in colloquial speech for a number of other adverbs, though. An example is the opposite of fast: slow. "Go slow" is often heard instead of "Go slowly".

I just want to step in to say that I’m 70% sure (only because anytime I say 100% I end up eating my words in some exception in another dialect of an English-speaking area) that “soonly” is not a word in the English language, or at least not in the eastern U.S. I’m 100% sure that I’ve never heard it here ever.

Because "fastly" and "soonly" are not proper adverbs. Fast's adverbial form is "fast", and soon itself is an adverb, so suffixing it with "-ly" is not necessary.

You speak English goodly! 

Don't say goodly either: it's "well"

You do it fast, you do it soon. Most *but not all* adverbs end in -ly. Fast does not require the -ly suffix for its adverb form. Soon is always an adverb, as it always describes when something is happening. So the adverb forms of fast and soon, are fast and soon.

A lot of people leave the -ly off quick and say/write something like ‘Let’s do it quick!’ I see this a lot in my work as an editor. That specific sentence does make the error a bit more obvious, but there are some constructions where it doesn’t seem so obvious at all. (And in dialogue, of course, it might actually be what the character would say, so it’s not ‘wrong’ per se in that context - although it’s still technically an error.) It would be considered quite pedantic to correct it in speech.

Since I haven't seen it in the top few posts... "Let's do it quickly" and "Let's do it fast" have essentially the same meaning. There may be some tiny nuances*, but most people don't think about them. Both are usually talking about the speed of completing a task. If you complete it in a total of 15 minutes, it is faster / quicker than if the task takes an hour. The opposite of this is "Let's do it slowly." *One of those nuances could be talking about the speed of your movements while completing the task. But this is almost never what people are thinking / talking about when using this phrase. "Let's do it soon" means something different entirely. It is talking about how much time between now and beginning a task. If I start a task in 1 hr, it is sooner than if I start it next week. The opposite of this is "Let's do it later" or "Let's wait to do it."

Because fastly is not a word. The adverb form of fast is still fast. So you could say "Let's do it fast." Soon is already an adverb, so there's no need to add "ly" to the end. So you'd say "Let's do it soon." "-ly" is added to many adjectives to make them into adverbs. But there are exceptions (as above with fast) and words that are already adverbs don't add "-ly" to them.

Neither of those is actually a word.

in addition to what others have said, English has a tendency to treat certain adjectives like adverbs. Such is the case with "quickly," even! "Let's do it quickly" and "Let's do it quick" are both grammatically sound.

“Quick” refers to the speed of an action over time, “soon” refers to a point of time in the near future, and “fast” refers to physical speed. You will hear people use “quick” and “fast” interchangeably sometimes, but these are the correct meanings of the words. As an example: a car is fast, I am going to bed soon, and I run fast. “Fastly/soonly” are also not words, although I am not sure why.

Let's do it soon. Let's do it quickly. Let's run fast.

...You know, I have no idea. It's just one of those things I learned as a kid and didn't question it. So it's probably arbitrary, that's just how the language evolved.

You could says “let’s do this fast/soon” but not “fastly/soonly”. “Let’s do this quickly” is correct. It’s to do with the adverbs.

You can say "lets do it soon" but it's not necessarily the same as "let's do it quickly." "Let's do it soon" mean it starts soon, but might not be finished soon, it could take a very long time. Like a second date might be "soon" but take hours if it goes well. Doing something quickly *might* start soon or might not, but from start to finish it doesn't take long. Like you might say "Let's do it quickly" in regards to cleaning the house. If you tell a date "Let's do it quickly" they think you don't want to spend time with them.