i find it easy to learn english.翻译:我发现学习英语很容易。
it做find的形式宾语,easy to learn english是宾语补足语;用it's也行,但是句子结构改变了,find后接的是that(被省略)的宾语从句
I find it easy to learn English. (it是find的形式宾语)
I find it is easy to learn English.(是省略句,省略了that,完整的是 I find that it is easy to learn English.)
it可以做主语,it's不可以做主语。