I think it interesting to study English,it后面可以不用is吗?

可以
两种都是对的,不过就是运用的语法点不一样而已
I think it interesting to study English.
主语 I
谓语 think
形式宾语 it
宾语补足语 interesting
真正的宾语是 to study English
I think it is interesting to study English.
主语I
谓语 think
后面是it is interesting to study English宾语从句,不过省略了that
而这宾语从句中
形式主语 It
谓语 is
表语 interesting
真正主语 to study English

要加is吧，完整的是I think that it is interesting to study English.没有is的话后面从句不完整了

不可以的，I think (that) it is interesting to studay English.
it is interesting to studay English是一个句子。