Words in English that can function as both adjectives and adverbs

 Fast: He ran fast (adverb), He is a fast runner (adjective).

Hard: He hit the ball hard (adverb), The rock is hard (adjective).

Close: She stood close to him (adverb), The store is close (adjective).

Early: He woke up early (adverb), It's early in the morning (adjective).

High: The plane flew high (adverb), The mountain is high (adjective).

Late: She arrived late (adverb), It's getting late (adjective).

Near: She lives near the park (adverb), The nearest grocery store is two blocks away (adjective).

Slow: He walked slow (adverb), The slow train was delayed (adjective).

Tight: He held on tight (adverb), The shirt is too tight (adjective).