Question

it is difficult to do和it difficul to do的区别详细一点的

Answer 1

it is difficult to do 是一个句型，其中 it 是形式主语，不定式 to do 是句子的真正主语。
如：It's difficult to learn English.

it difficul to do 则只是一个结构，其中 it 一般视为形式宾语，形容词 difficult 是宾语补足语，不定式 to do 则是真正的宾语。该结构前面一般都有一个及物动词如 think, believe, find, make 或者 consider 等等。
如：I think it difficult to learn English.

追问

那如果将it is difficult to learn english写成主句为I think 的宾语从句且省略引导词that应该怎样区分?

追答

很简单啊
it is difficult to learn English 做宾语从句啊
it是宾语从句的形式主语，to learn English 则是宾语从句的真正宾语。
而 如果是 I think it difficult to learn English.
it 是形式宾语，to learn English 是真正的宾语
形容词 difficult 则是宾语补足语

Answer 2

拿一个例子：1.I find (that) it is difficult to get good grades.
2.I find it difficult to get good grades.
前面是从句，后面是一个固定用法。it is difficult to do前面要接that(也可以省略）

追问

那省略that时如何判断填it还是it is?