第三题第三句不能用where于逗号去不去没有关系。因为这句话想表达的是:这间屋子的窗户朝东。屋子的窗户(the window of the room) ,用which 代替the room,得:the window of which ,所以句子可以写成:He still lives in the room, the window of which faces to the east. (第二句),再把介词加关系代词调到the room 后得:He still lives in the room, of which the window faces to the east.
第四句中in the middle of 是介词词组,等于一个介词的用法只能用关系代词which 。