This is the 8195th most frequent Korean word.
in
The Korean word "in" (인) can mean "person," "human," or "individual," depending on the context.
Here, '있는' (in) represents the state or position of the book within the school.
나는 학교에 있는 책을 가져왔어요.
I brought the book that is in the school.
In this case, '속에서' (in) illustrates the location of the action within the water.
그녀는 물 속에서 수영을 해요.
She swims in the water.
'마음 속' (in the mind) depicts an abstract conceptual interior where thoughts are situated.