People know that he has trained very hard. He.... / It...
He is known to have trained very hard. / It is known that he has trained very hard.
They say that Emma plays the violin very well. Emma... / It...
Emma is said to play the violin very well. / It is said that Emma plays the piano very well.
People think that pirates hid treasures in the Caribbean. Pirates.... / It...
Pirates are thought to have hid treasures in the Caribbean. / It is thought that Pirates hid treasures in the Caribbean.