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.
(..) Yes. How many fingers am I holding up? Five. You see, at least, that it is possible. You've understood the reality of the past and the present, Winston. Now, what of the future? A question: how does one man assert power over another? - By making him suffer. - Exactly. Obedience is not enough. (..)1984 1984