Flat Adverbs
Flat adverbs are established lexical patterns, not a productive rule that turns every adjective into an adverb. Some pairs coexist with little difference, while others differ in meaning or register.
A flat adverb has the same form as its related adjective, without an added -ly, as in established uses such as “drive fast” or “work hard.” Whether a flat form is standard, informal, dialectal, or meaningfully different from an -ly form depends on the individual word.
Key details
Further guidance
Contrast
Some forms contrast in meaning: an established flat adverb can behave differently from its -ly counterpart, so substitution is not automatically safe.
Decision Path
Check the particular word and intended meaning rather than applying a blanket “drop -ly” rule.
Register
A flat form may be standard in one construction but informal, dialectal, or uncommon in another.
Worked Example
“The train moved fast” uses established adverbial “fast”; there is no standard “fastly” equivalent for that meaning.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- The Adverb: A Most Fascinating POS — Flat Adverbs (opens in a new tab)Merriam-Webster · Defines flat adverbs as sharing the form of related adjectives, explains their history and competition with -ly forms.
- Fast, quick or quickly? (opens in a new tab)Cambridge University Press · Shows a current flat-adverb example: fast functions as both adjective and adverb, while quick requires quickly in adverb use.