"VERB which ends with 하다" - "다" + "ㄴ" becomes "adjective". for instance; 상하다(to go rotten) - 다 + ㄴ = 상한(rotten) for such a reason, "VERB which ends with 엽다" - "ㅂ다" + "운" becomes "adjective" too. examples; 귀엽다(to be cute) - ㅂ다 + 운 = 귀여운(cute) 어렵다(to be difficult) - ㅂ다 + 운 = 어려운(difficult) hope it helps

"어렵다" is "to be difficult" not just "difficult(=어려운)".^^/ Adjectives in Korean also end with '다' like verbs, so we call the dictionary forms of the adjectives "descriptive verbs" to tell between "to be + adjective" and "adjective". To make these descriptive verbs into the adjectives, we need to change them to the "-(으)ㄴ" forms. Usually this form needs when the descriptive verbs come before nouns. For example, The verb stem of 어렵다 is "어려". 어렵다 = 어려+ㄴ = 어려운 Likewise, 쉽다(to be easy) = 쉬+(으)ㄴ = 쉬운(easy) 맛있다(to be delicious) = 맛있+(으)ㄴ = 맛있는(delicious) 예쁘다(to be beautiful) = 예쁘 + (으)ㄴ = 예쁜(exception) 좋다(to be good) = 좋 + (으)ㄴ = 좋은(good) 행복하다(to be happy) = 행복하 + (으)ㄴ = 행복한(happy) 어렵다 + 문제(problem - noun) = 어려운 문제 쉽다 + 문제 = 쉬운 문제 맛있다 + 음식(food - noun) = 맛있는 음식 \^o^/