it difficult to do sth 和it is difficult to do sth 的区别

it difficult to do sth: difficult修饰to do sth。例如：
I feel it difficult to speak English.
其中it指代speak English,difficult修饰speak English。

It is difficult to do sth:常见句型，意思是:做什么事...难。

it difficult to do sth不完整吧，这个结构只有主语成分，不能成句子，其中difficult to do sth是省略的定语从句，修饰it的