Introductory it as Subject
Pattern group 4: it be V-ed clauseThere are three patterns in this group, each consisting of it, a verb in the passive, and a type of clause. This pattern is always passive; there is no active equivalent containing it.
4.1 it be V-ed thatIt is thought that the temple was used in the third century.
4.2 it be V-ed to-infIt was decided to cancel the meeting.
4.3 it be V-ed whIt is not known what causes the disease.