This is the 1484th most frequent Korean word.
하나가
"하나가" means "one is" or "one has" depending on the context.
In this instance, '하나가' denotes a single problem among multiple problems being resolved.
문제들 중 하나가 해결되었습니다.
One of the problems has been resolved.
In this sentence, '하나가' is used to emphasize the single item being discussed as important.
이 하나가 정말 중요합니다.
This one is really important.
Here, '하나가' refers to an individual out of a group performing an action.