This being ...这是一个独立主格结构.
如果去掉This,句子就变成：Being his first visit there,he found ...
这时,he就成为being这个分词的逻辑主语了,但是,he和being不能构成逻辑上的主语,这个句子是不成立的.
解决的办法之一就是在分词前加上它本身自己的主语,即构成独立主格结构了.如本句话,前面半个句子实际上是This was his first visit there,因此就变成了This being his first visit there了.
独立主格结构有两部分组成,前一部份是名词或者代词,后一部分是非谓语动词.前后两部分具有逻辑主谓关系.独立主格结构在句中做状语,多用于书面语.独立主格结构本身不是句子,在句子中作状语,表示时间、原因、条件、伴随、目的等,相当于一个状语从句或并列句.
题目中的句子,独立主格表示的就是原因,可以翻译为：这是他第一次去那儿参观,他发现有很多他感兴趣的东西.
再如：It being Sunday (= Because it is Sunday),we needn't go to school.
The work done(=After the work had been done),we went home.
Weather permitting (=If weather permits),they will go to the beach tomorrow.
He was lying on the grass,his hands crossed under his head (=and his hands were crossed under his head).