新概念3英语语法问题.Lesson 49After her departure,Aunt Harriet discove
2个回答

in 后面是一个定语从句.

what=the thing which,what在这里是一个复合关系代词.

in what had once been Bessie's wardrobe=in the thing which had once been Bessie's wardrobe

the thing做介词in的宾语,从句which had once been Bessie's wardrobe做the thing的定语.