形式宾语it的用法

Question

they found it difficult to study well 与 they found it is difficult to study well 为什么第二个句子不对。如果把第一句study well代替it，把后面的to study well去掉，再读，好像应该加上个is在difficult前面句子才成立？？？

Answer 1

第二个句子不对，是因为不应该用is,应该用was,因为前面是found,要保持时态一致，如果前面是find，用is就对了。
如果把第一句study well代替it，把后面的to study well去掉，也不应该加is，而是was，道理是一样的，因为此刻句子是宾语从句，省略了that，完整的句子应该是They found (that) study well was difficult.

Answer 2

They found it.

However, what is it? The statement （句子） contains ambiguity （意义不明确）.
Therefore, we add something to tell something more specific about "it".

Now, you know "difficult to study well" is to describe the object "it".

Or to be clumsy enough,
They found (that) it *was* difficult to be studied well.