Flat Adverbs
Adverbs that are unchanged from their adjective form are known as "flat" adverbs.
Take the following sentence:
He kicked the ball high. The high ball slammed into the net.
The first use of high is a flat adverb, affecting the verb kicked. The second use of high is as an adjective, affecting the noun ball. To novice English speakers, it may seem more logical to say: He kicked the ball highly. Alas, English is rarely a language of logic!
Other words that are commonly used as flat adverbs include low, early, late, near, far, long, and short. Many idioms also introduce a number of non-standard flat adverbs into everyday use such as in wax lyrical, something rotten, and something awful.