This is the 2345th most frequent Korean word.
것만
"것만" means "only that" or "just that" in English.
The expression '것만' here emphasizes exclusivity, where '것' relates to the act of thinking and '만' reinforces the idea of exclusivity only to her happiness.
그녀는 자신의 행복만 생각하는 것이 문제예요.
The issue is that she thinks only about her own happiness.
In this sentence, '것만' refers to the action or deed of striving, and '만' indicates that this action alone is insufficient.
성공을 위해 노력하는 것만으로는 충분하지 않아요.
Simply striving for success is not enough.
Here, '것만' is used to imply exclusivity related to the items being chosen, with '만' emphasizing the selection limited to good items.