The most common Korean word for “hard” is 힘들다 (himdeulda). There is one more word with a similar meaning — compare them below. 「hard」는 한국어로 힘들다.
to be hard; to be difficult
TOPIK 1 · adjective
Words and levels: TOPIK I essentials (National Institute of Korean Language standard curriculum, KOGL Type 1) · examples: Cheesepotato