마음+만 = ma-eum-man = only 마음 마음+을 = ma-eum-eul = 마음을 (it can be an object, 마음을 needs a subject and verb to make sense) 마음+속에 = ma-eum-sok-e = inside(=속에) 마음 = in one's mind or at hearts. \^o^/