He spoke quietly. They shivered violently. She ran quickly. Adverbs are a handy writer’s tool to describe an action, derived as a compound of adjective + verb. We're usually able to spot adverbs easily, as most end in -ly, as in the above examples.
So what's a flat adverb? A flat adverb refers to a describing word that’s missing that -ly suffix. They're a little trickier to find, and can be confused with adjectives. Here's where you can spot flat adverbs and tell them apart from other describing words.
Flat adverbs often look identical to their adjective counterparts — both are used to describe and drop the -ly suffix. But adverbs describe a verb instead of a noun. Seems simple enough, but many adverbs and adjectives are interchangeable.
Take for example, the word fast. It's both an adjective and a flat adverb. As an adjective, you might say, “She was on the fast track to success.” As a flat adverb, the meaning changes slightly when you write, “She ran fast.”
To confuse the issue, fastly is actually a word; though not one we use in modern-day language.
To make it even more confusing, 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, these 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 adverb definition. The good news is, you can use either version – soft or softly – and generally get your message across, although it should be noted that flat adverbs do tend to have a more casual vibe.
Interestingly, there are still other adverb/adjective pairs where the meaning actually differs between them. For example, late and lately have two different meanings – you can arrive late, or you can not have stopped by lately.
All of these divergences led 18th-century grammarians to actually eschew the flat adverb, suggesting it was a mistake. But in modern-day linguistics, it’s commonly understood that flat adverbs are okay to use – if the meaning of your sentence doesn’t radically shift. As a general rule, most of us are so used to that -ly ending that if it makes sense to use it, go for that ending. “Go quickly,” is a much more common sentence than “Go quick,” for example.