Question

I found it impossible to answer all the questions.

请问为什么不是found it is impossible?谢谢

Answer 1

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。

Answer 2

find sth adj. 这是个固定用法...
find （that） it is impossible相当于是find后面接了从句....

二者皆可....不过第二种比较拖沓，一般不用

Answer 3

it作形式主语

Answer 4

I found it impossible to answer all the questions
I found (that) it was impossible to answer all the questions.
两句意思一样。

Answer 5

这个句型死记就行