Question

一个英语句型，麻烦高手分析一下，谢谢。

But it is a little upsetting to read in the explanatory notes that a certain line describes a fight between a Turkish and a Bulgarian officer on a bridge off which they both fall into the river
请问这个是什么结构，其中it指代什么，that是同位语吗？

Answer 1

explanatory notes 是注释说明的意思
注释说明中有那么一行专门描述两个分别来自土耳其和保加利亚的军官在一座桥上打架结果掉到河里的事，读到这儿时，真让人觉得苦恼\沮丧。（总的是个形式主语和真主语的问题，it 是形式主语；that 是read 的宾语）
分析如下：
主干：it is a little upsetting to read that...
宾语从句：that a certain line describes a fight between a Turkish and a Bulgarian officer on a bridge off which they both fall into the river
介词+关系代词 引导的定语从句：off which they both fall into the river（which 指bridge桥）
in explanatory notes 是表示地点的介词短语，指所读内容的出处

Answer 2

if 指代 to read in the explanatory notes ，这种结构很常见的，文章里面非常典型的句子
that 是 note的内容，是同位语吧，
要想熟练看出这些句子，只能多读书，而且要读点有难度的，直接一看就懂的读了没意义。

Answer 3

it 是形式主语吧，因为真正的主语后置了，应该是to read in the explanatory notes balabala is a little upsetting 。that 是修饰read的宾语从句，后面which不是定语从句么，先行词是bridge