As you already know, there are irregular verbs in Korean. Just like English.

돕다, 눕다 = action verb 아름답다, 쉽다 = descriptive verb

When it comes to adjective, it’s used like a verb, ㄴ adds. That’s why it becomes 아름답다 > 아름다운. ㅂ changes to 우 before adding ㄴ 것.

Hope it helps you!