This is the 4111th most frequent Korean word.
적응
Adaptation.
In this sentence, '적응' is used as a noun meaning 'adaptation,' indicating the process of adjusting to a new setting.
새로운 환경에 적응하기 어렵습니다.
It's difficult to adapt to a new environment.
In this case, '적응' combines with '-하다' to form the verb '적응하다,' which means 'to adapt.' It describes the action of adjusting oneself to the strenuous conditions.
그는 고된 훈련에도 적응했습니다.
He adapted even to the rigorous training.
Here, '적응' is again used as a noun to refer to adaptation as a concept or quality necessary for success.