I found it impossible to answer all the questions.

题目

I found it impossible to answer all the questions.
请问为什么不是found it is impossible?

答案

find + it+ adj/n.+to do .是个固定句型：“动词+ it（形式宾语）+ 形容词/名词（宾补）+ to do (真正的宾语）...(能这样用的动词还有：think,feel,make etc)
再如：I find it hard to learn English .
I feel it my duty to help you.
find后面也可以接从句,即你提到的这种结构：find (that) it is impossible to do.上面你所给的这个句子就可以转化成：I found (that) it was impossible to answer all the questions.,不过不同的是,不是is 而是was,因为主句时态用了一般过去时,所以从句时态也须使用过去时态.
不过第二种结构不够简洁.