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.