When people say “they're engaged,” the phrase “to be married” 'is understood'. [=“they're engaged” is understood to mean “they're engaged to be married”]
We still don't fully 'understand' the causes of the disease. = The causes of the disease are still not fully 'understood'.
He believed it 'was understood' [=believed that everyone had agreed] that he would be paid for the work.
(..) See? Thank you, Alfredo. - Good-bye. - Bye. See you. l understood right away. The square is mine! lt's my square! lt's midnight. Midnight! Out! Shoo! l have to close the square. (..)1988 Cinema Paradiso