Flat adverb

In English grammar, a flat adverb or bare adverb is an adverb that has the same form as a related adjective. It doesn't end in -ly, e.g. "drive slow", "drive fast". A flat adverb is sometimes also called simple adverb, but this can mean any single-word adverb.

Despite continued fairly widespread use in speech, only a few flat adverbs that have a possible alternative form ending in -ly are still in formal use, and even these are often considered incorrect due to a widely taught and widely believed incorrect rule.

So despite bare adverbs being grammatically correct and widely used by respected authors, it can be advisable to instead use the forms with -ly because these are often considered more socially accepted and of higher status. There have recently even been public campaigns of citizens against street signs with the traditional text "go slow" (which has already been used for generations) and the innovative text "drive friendly" despite both being grammatically correct. In the latter case, the campaign even demanded that the correct bare adverb friendly be replaced with the extremely rare and clumsy form friendlily.

For most bare adverbs, there exist the alternative form ending in -ly (slowly), but this form doesn't exist for some bare adverbs (e.g. fast, soon, straight, tough, far), sometimes this form has a different meaning (hardly, nearly, cleanly, rightly, closely), and sometimes this form is not used for certain meanings (sit tight, sleep tight). In addition, the ending -ly is also found on some words that are both adverbs and adjectives (e.g. friendly) and some words that are only adjectives (e.g. lonely).

Flat adverbs were once quite common but have been largely replaced by their -ly counterparts. In the 18th century, grammarians believed flat adverbs to be adjectives, and insisted that adverbs need to end in -ly. According to the Merriam-Webster Dictionary, "It's these grammarians we have to thank for ... the sad lack of flat adverbs today".

Nonetheless, flat adverbs are still preferred for certain meanings (and such use is often called idiomatic because of the still strong influence of the incorrect rule of the confused 18th-century grammarians), as in "take it easy" and "sleep tight".

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Wikipedia