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