我给你演示一下句式的转换：

1 They/People say: "One million people are learning English now." 直接引语

2 They/People say that one million people are learning English now. 间接引语

3 It is said that one million people are learning English now. 被动语态

4 One million people are said to be learning English now. 提升主语结构

说明：

句3 的宾语为that从句。从句作宾语改为被动语态句子时，不能直接用that从句作主语，而必须采用外置主语结构，用形式主语 it 作主语。

句4 是由句3 改变而来的提升主语结构。所谓提升主语结构，指的是一个从句的主语提升到主句作主语，原从句的谓语动词改为不定式作主句中的主语补足语。主句的原谓语动词不变，只做动词数的调整。即删去句3的形式主语it 和引导宾语从句的连词that，用that从句的主语one million people 作主句be said 的主语，由于主语为复数名词，is said改为are said。原从句的谓语部分are learning English now 改为不定式短语 to be learning English now，与one million people一起构成复合主语。结果就得到句4。

5 They/People say: "There are one million people learning English now. " 直接引语

6 They/People say that there are one million people learning English now. 间接引语

7 It is said that there are one million people learning English now. 被动语态

8 There are said to be one million people learning English now. 提升主语结构

句5 到句 8 的演变与句1 到句 4 的演变完全相同。只不过直接引语用了there be存在句，以避免用泛指名词（新信息）one million people作主语。在there be存在句中，there是形式主语，起着语法上的主语的作用。所以在句 7 转变为句 8 时，that从句的形式主语there 提升到主句作主语，由于真正主语为复数，因而there也是复数意义（there是单复数同形的代词），所以 is said 改为 are said. there提升为主句的主语后，there后的are one million people learning English now 改为不定式结构 to be one million people learning English now，与there一起构成复合主语。 这样就得到了句8.

这就是网友这个句子的由来。也能说明为什么不能改为 It is said...