你好。给你一个句型转换,这样你就全部明白了。
They say that Mary is very happy in London---------
宾语从句===It is said that Mary is very happy in London ---------
主语从句,it是
形式主语,that是主语从句的连接词,Mary--- - - - .是真正主语
=== Mary is said to be very happy in London.--------简单句,里面的to be very happy in London是主语补足语。本回答被提问者采纳