수동태 문제에서 두 문장 구조가 헷갈려요

독학 중인데 능동태 문장을 수동태로 바꾸는 부분에서 막혔어요.
“They say that baseball was invented in New York in 1839”를 “It is said that baseball was invented in New York in 1839”와 “Baseball is said to have been invented in New York in 1839”로 바꾸는 이유를 설명해 주실 수 있나요?