He spoke quietly. They shivered violently. She ran quickly. Adverbs are a handy writer’s tool to describe an action, or give more detail about a verb. We're usually able to spot adverbs easily, as most end in "-ly."
But not always! A flat adverb refers to a descriptive word that’s missing that "-ly" suffix. They're a little trickier to find, and can be confused with adjectives. Here's how you can spot flat adverbs and tell them apart from other descriptive words.
Adverb vs. Adjective
Flat adverbs often look identical to their adjective counterparts — both are used to modify or describe another part of speech. But 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 both an adjective and an adverb. As an adjective, you might say, “She was on the fast track to success.” As a flat adverb, the meaning changes when you write, “She ran fast.”
To confuse the issue, there are regular adverb synonyms ("quickly," "swiftly," "hurriedly") for the flat adverb "fast."
Flat Adverb/Adjective Pairs
There are some traditional adverbs that have a flat adverb mate, such as "soft" and "softly," "quick" and "quickly," or "bright" and "brightly." How did these come about?
In Old English texts, adverbs usually had an inflection at the end of them — like brighte. When these were dropped over time, they were more easily confused with their adjective counterparts, so "-ly" was added to help clarify when you were intending to use the adverbial form. The good news is, you can use either version —"soft" or "softly" — and get your message across, although it should be noted that flat adverbs do tend to have a more casual vibe.
However, there are other adverb/adjective pairs where the meaning differs between the two words. For example, "late" and "lately." You can arrive late, or you can not have stopped by lately.
If you've been told that using a flat adverb like "fast" was a mistake, you can blame 18th-century grammarians for that. But in modern linguistics, it’s commonly understood that flat adverbs are OK to use — if the meaning of your sentence doesn’t radically shift. But as a general rule, most of us are so used to that "-ly" ending that it just comes naturally.