分析句子结构: I found it impossible to do it..

i是主语
found 是谓语
it 是形式宾语
impossible to do it 才是真正的宾语

it形式宾语的考点说明

通常意义的形式宾语主要见于真正宾语为不定式、动名词、从句等且其后跟有宾语补足语时的情形，即用于“动词+it+宾语补足语+真正的宾语”这样的结构，但以下两类形式宾语却比较特殊，其后没有宾语补足语。如：

(1) 由于介词后通常不能直接跟that从句作宾语，有个别特殊结构就在介词后先接it作形式宾语，再跟that从句。如：

You may depend on it that he will turn up in time. 你可以相信他会及时赶到的。

I can’t answer for it that the boy is honest. 我不能保证这个男孩是诚实的。

I’ll see to it that all these letters will be sent to the post before twelve. 我将注意让所有的信件在12点以前送到邮局。

【注意】① 在其他情况下解决介词后跟that从句的办法是借用the fact作宾语。如：

They were worried over the fact that you were sick. 他们为你生病发愁。

② 个别介词(主要是but和except)也可接that从句作宾语。如：

I know nothing about him except that he lives next door. 我只知道他住在隔壁，其他的就不知道了。

(2) 有的动词由于通常只用作及物动词，当它们后接if从句或when从句，通常应先在动词后接it作形式宾语。如：

She won’t like it if you arrive late. 她不喜欢你迟到。

He hates it when people use his bike. 他讨厌别人用他的自行车。

I hate it if you say such things in public. 我讨厌你在大庭广众之下说那样的事。

I’d prefer it if I didn’t have to do so much work. 要是我不必做那么多工作，那就太好了。

I would appreciate it very much if you would help me with it. 如果你能帮助我做这事，我会十分感激。

【注意】能这样用的动词不多，主要的有enjoy, like, dislike, love, hate, prefer, appreciate等表示情绪的动词。而且高考对此也有所考查，请看一道高考真题(答案选A)。如：

I hate ______ when people talk with their mouths full.

A. it B. that C. these D. them

(3) 用于 have it that(说，认为)，take it that(认为)等个别固定表达中。如：

I take it (that) you agree. 我以为你同意了。(from www.46english.com)

Report has it that about 30 people were killed in the accident. 据报道，在这次事故中大约有30人丧命。

it impossible to do it做found的宾语 to do it 动词不定式是it impossible to do it里面的宾语。

主语：I

谓语：found

it：形式宾语 代指后面的 to do it

to do it 是真实宾语

impossible：宾补。。。

impossible是修饰it的。
后面的部分改过来是 It is impossible to do it.
impossible是来形容to do it的