Some words can be used both as adjectives and adverbs. Some common examples of such words are given below.

Loud

She spoke in a loud voice. (Here the adjective loud modifies the noun voice.)
Why are you talking so loud? (Here loud is used as an adverb. Loudly is also possible here.)

Fast

India does not have many fast bowlers. (fast – adjective)
He drives very fast. (fast – adverb)

Back

The thieves escaped through the back door. (back – adjective)
Go back. (adverb)

Little

Even little difficulties upset him. (little – adjective)
He is little known outside India. (little – adverb)

Hard

He is a hard nut to crack. (hard – adjective)
Work hard. (hard – adverb)

Quick

He is quick to take offence. (quick – adjective)
Run quick. OR Run quickly. (Quick – adverb)

Early

The early bird catches the worm. (early – adjective)
We started early. (early – adverb)

Only

He is the only child of his parents. (only – adjective)
I could only scream. (only – adverb)

Enough

We have enough food to last a week. (enough – adjective)

She runs fast enough. (Here the adverb enough modifies the verb run.)

Better

He is a better bowler than you. (better – adjective)
I can do it better than you. (better – adverb)