As an adverb, slow
has two forms, slow
and slowly. Slowly
appeared first in the 15th century; slow
came into use shortly thereafter. Both are standard today in certain uses.
was used both preceding and following the verb it modified. Today, it is used chiefly in imperative constructions with short verbs of motion (drive, run, turn, walk,
etc.), and it follows the verb: Drive slow. Don't walk so slow.
This use is more common in speech than in writing, although it occurs widely on traffic and road signs. Slow
also combines with present participles in forming adjectives: slow-burning; slow-moving.
In this use it is standard in all varieties of speech and writing. Slowly
is by far the more common form of the adverb in writing. In both speech and writing it is the usual form in preverb position (He slowly drove down the street. The couple slowly strolled into the park
) and following verbs that are not imperatives (He drove slowly down the street. The couple strolled slowly through the park
). See also quick