Your question is related with ㅂirregular verb/adjective and verbal adjective ending. You must remember following three rules. 1. ㅂirregular verb/adjective rule is: When a word stem with final consonant ㅂ + a particle start with vowel, ㅂ changed 우 or 오. * ㅂ => 오 (only with 아 vowel) => 와 돕다, 곱다. 돕(다) + 아요 => 도와요 곱(다) + 아요 => 고와요 돕(다) + 으면 => 도우면 곱(다) + 으면 => 고우면 * ㅂ => 우 어렵다, 쉽다, 덥다, 춥다, 귀엽다, 입다, 아름답다, 맵다, 눕다, 줍다. 덥(다) + 어요 => 더워요 NB. 입다, 잡다, 씹다, 좁다, 넓다, 뽑다, 접다 are not ㅂirregular verb/adjective 2. Verbal adjective ending for verb have four tesnses * present -는 저기 가는 자동차 * past (으)ㄴ 어제 먹은 빵 * incomplete past -던 어제 먹던 빵 * future -(으)ㄹ 내일 먹을 밥 3. Verbal adjective ending for adjective have three tenses * present -(으)ㄴ 예쁜 꽃 * past -던 예뻤던 꽃 * future -(으)ㄹ 예쁠 꽃 Now analyze each words. 어려운 것 is composed with 어렵(다) + (으)ㄴ + 것 어렵다 is ㅂirregular adjective. 어려운 is present tense of verbal adjective ending of 어렵다 돕는 것 is composed with 돕(다) + 는 + 것 돕다 is ㅂirregular verb. 돕는 is present tense of verbal adjective ending of 돕다 cf) 도운 것 is composed with 돕(다) + (으)ㄴ + 것 돕다 is ㅂirregular verb. 도운 is past tense of verbal adjective ending of 돕다 굽는 것 is composed with 굽(다) + 는 + 것 굽다 is ㅂirregular verb. 굽는 is present tense of verbal adjective ending of 굽다 미운 것 is composed with 밉(다) + (으)ㄴ + 것 밉다 is ㅂirregular adjective. 미운 is present tense of verbal adjective ending of 밉다 매운 것 is composed with 맵(다) + (으)ㄴ + 것 맵다 is ㅂirregular adjective. 매운 is present tense of verbal adjective ending of 맵다

it's a tough question to answer. I am not sure if there's any rules to explain. If there is, I believe some other Koreans would explain to you. In my opinion, you just have to memorize each case. Sometimes you just speak that way. I know that English is also like that sometimes, right? But I could be wrong. If I am, please correct me.