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.
(..) Oh heavy deed. It had been so with us had we been there His liberty is full of threats to all. To you yourself, to us, to everyone And pray, how shall this deed be understood? It will be laid to us, whose providence... ...should have kept short, restrained and out of haunt this mad young man (..)2015 Hamlet