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.
(..) and all the realms with my last and every breath, but I cannot do so from that chair. Loki, for all his grave imbalance, understood rule as I know I never will. The brutality, the sacrifice... It changes you. I'd rather be a good man than a great king. Is this my son I hear, (..)2013 Thor: The Dark World