This is the 11557th most frequent Korean word.
나서야
"나서야" means "only when (someone) steps forward or takes action."
나서야 is used to indicate an action requires stepping forward or taking the initiative in a situation.
나는 친구가 아플 때 나서야 도와줄 수 있었다.
I could help my friend only when I stepped forward while they were sick.
나서야 is emphasizing the necessity to take action after a specific condition is met.
문제가 발생한 후에야 나서야 했다고 느꼈다.
I felt that I should have stepped forward only after a problem occurred.
나서야 here imparts the idea that taking initiative is a prerequisite for making progress or understanding further steps.