it是代词作主语，它所代替的是方式状语the way he keeps changing his mind的内容。又如：

It's disgraceful the way he treats his mother.他那样对待他母亲太不像话了。《牛津高阶英汉双解词典》

英语语法网上有刘永科老师写的一篇it代替状语从句的内容的文章：

http://www.yygrammar.com/Article/201501/3810.html

▲the way可以看作引导从句的连词。费致德《现代英语惯用法词典》指出：

美国人把the way用为副词对格，等于in the way (manner) in which；有时去掉in the way的in，也有时用为直接宾语。含义如下：

1. the way = as

I fixed things the way he wanted. 我按照他的要求的样子把东西搞好了。

2. the way = according as

We must do the way Chairman Mao teaches us. 我们必须按照毛主席教导的那样做。

3. the way = how

Mrs Collion's eyes filled. She said, "No one except you, not even the children can understand the way I miss David."柯林斯太太眼睛充满了泪水。她说：“除你以外，没有人理解我是多么地想念大卫，甚至那些孩子们也不理解。”

下面的句子是2007年山东卷的一道高考题句子：

It worries me the way he keeps changing his mind.

我对这个句子不是很理解，尤其是这里的 it 是什么东西？难道是形式主语吗？

如果这个 it 是形式主语，那真正的主语呢？是名词 the way 吗？名词作主语也可以用形式主语吗？

同意曾老师解答

it代替+the way(that/in which) he keeps changing his mind 整句话内容

It worries me the way he keeps changing his mind.

=It worries me the way in which he keeps changing his mind.

=It worries me the way that he keeps changing his mind.

=It worries me  how he keeps changing his mind.

it代替后面从句the way in which he keeps changing his mind