쉽다 and 어렵다 are both ㅂ irregular adjectives. When a word stem with final consonant ㅂ + a particle start with vowel, ㅂ changed 우 before vowel sound of not 오. 쉽(다) + 어요 => 쉬우어요 => 쉬워요 it is easy 어렵(다) + 어요 => 어려우어요 => 어려워요 it is difficult 쉽(다) + 었 +어요 => 쉬우었어요 => 쉬웠어요 it was easy 어렵(다) + 었 + 어요 => 어려우었어요 => 어려웠어요 it was difficult Most verbs/adjectives with last consonant ㅂ are irregular. But 입다, 잡다, 씹다, 좁다, 넓다, 뽑다, 접다 are not ㅂirregular verb/adjective For more information, refer to http://www.italki.com/question/256216