he is finding it difficult to get his room in order.为什么it 后面

这是“形式宾语”,
这句话本来的结构应该是：
he is finding /to get his room in order/ difficult.
不定式短语to get his room in order作find的宾语,
difficult是宾语补足语,
但是不定式作宾语(或主语)时,往往用it代替不定式作宾语(或主语),而不定式本身后置,这时候的it被称为“形式宾语(或主语)”
如果加上it,就变成：
he is finding it is difficult to get his room in order.
就变成it is difficult to get his room in order作宾语从句.
你应该把形式主语和形式宾语这部分看一下

如果在it 后面加is的话,一个简单句里就有两个谓语了!所以不能加is.出现这种原因,是因为你的语感造成了,你平时已经习惯"it is +形容词"的句子结构.
不过如果要加is的话可以变成宾语从句:
he is finding that it is difficult to get his room in order.

主要是句子的结构你没弄明白。我在这举几个例子把你的问题详细分析一下。
跟上面差不多的例子：
He finds it difficult to get his room in order.
这里有个find，这句话真正的句子应该是这样的：He finds to get his room in order difficult.就是一个很简单的find sth difficult...

其实it表示 to get his room in order。
也可以说he is finding that it is difficult to get his room in order.
这两个句型我记不得了！

Find 后：
1、可用分词做宾补；
I found him lying on the ground.
2、或先加形式宾语，再加形容词，最后加带to 的动词不定式；
I found it important to learn.
3、也可带一个从句。
I found that to learn English is important.
此类动词还有get，have。