It's difficult to do 还是 It's difficult doing

it's difficult to do这个才是正确的。

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.

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