mary is no longer the girl that she used to be 关系代词在哪里的从句中作表语

此句是一个由 that 引导定语从句，先行词是girl， that she used to be 是定语从句 that 作从句中的表语。

关系代词在从句中表语的时候不是很多
1.He is no longer the man （that） he was.他已经不是过去的他了.
2.He will become the man which the Party wants him to be.他将成为党所期望的那种人.
that在定语从句中作表语时的省略.
说明：关系代词作be 的表语,且先行词是特指时,关系代词通常用that(不用who或which)或省略
She was not the beautiful girl (that) she used to be.她已经不是过去那个漂亮姑娘了.
但如果先行词是泛指时,关系代词通常用which,且不省略：
He looked like a teacher which he was.他像个老师,而他也的确是个老师.