He spoke quietly. They shivered violently. She ran quickly. The last word in each of these three sentences is an adverb, a word that modifies a verb. We know that a complete sentence has a subject (a noun or a pronoun) and a verb, but adverbs are another part of speech that serves to add color and nuance to clauses. They're a handy writer’s tool used to describe an action, or give more detail about a verb. We're usually able to spot adverbs easily, as most end in the letters "-ly."
This is not always the case, though — a flat adverb is a descriptive word that's missing that "-ly" suffix. Flat adverbs are a little trickier to find and can be confused with adjectives. Here's how to spot them and tell them apart from other descriptive words.
Adverb vs. Adjective
Flat adverbs often look identical to their adjective counterparts, and both are used to modify or describe another part of speech. However, they differ in usage: Adverbs describe a verb instead of a noun. It seems simple enough, but many adverbs and adjectives are interchangeable.
Take, for example, the word "fast." It can be used as both an adjective and an adverb. To use it as an adjective, you might say, "She was on the fast track to success." In this example, the adjective "fast" describes the noun "track." When "fast" is used as a flat adverb, though, the meaning of the word changes. In the phrase "she ran fast," the flat adverb "fast" describes the speed of the verb "ran."
There are, of course, regular adverb synonyms ("quickly," "swiftly," "hurriedly") for the flat adverb "fast." Each of these conveys a slightly different tone than the flat adverb, which helps explain why there are so many more regular adverbs than flat. If the goal is to add description, it's usually worth using the most powerful word at your disposal.
Adverb Pairs
Some traditional adverbs have a flat adverb mate, including "soft"/"softly," "quick"/"quickly," and "bright"/"brightly." In Old English texts, adverbs usually had an inflection at the end of them (brighte). When these inflections were dropped over time, the words were more easily confused with their adjective counterparts, so "-ly" was added to help clarify when you were intending to use the adverbial form. Either version of most adverb pairs is accepted and understood, but it should be noted that flat adverbs do tend to have a more casual vibe.
However, there are a few adverb/flat adverb pairs where the meaning differs between the two words — for example, "late" and "lately." You can arrive late (meaning you're tardy), or you can apologize for not visiting lately (meaning it's been a long time since you've visited). Both are related to spans of time, but the two forms of the adverbs mean very different things.
If you've been told that using a flat adverb like "fast" was a mistake, that's a holdover from an 18th-century grammar dictum. In modern linguistics, it's commonly understood that flat adverbs are OK to use, as long as the meaning of your sentence doesn't radically shift.