新概念3英语语法问题.

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.
再问： 现在又发现了lesson 49中的另一个语法问题， 麻烦帮忙解析一下 :-)
During that time she so gained my aunt's confidence, that she was put in charge of the domestic staff. So 后面为什么用 gained ?? gained 是个形容词吗 ？ 因为So 后面只能用形容词啊。 求解~
再答： gained 是动词。
这里是so....that...引导的结果状语从句。
的确，So 后面多用形容词、副词，但字典里也没绝对说其后不可接动词的。
我以前看到这样的句子：He so adores his daughters that he keeps buying them expensive toys.

He so hated the woman that he killed her.

我认为so....that...引导的结果状语从句中so可以放在动词前的，不知道对不对。