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.
(..) You did terrible things. Really terrible things, Robert. And all for nothing. For nothing? Yeah. You never understood... ...why we did this. The audience knows the truth. The world is simple. It's miserable. Solid... Solid all the way through. (..)2006 The Prestige
(..) - The Great Danton. - Do you like it? But it's sophisticated. It's French. Borden writes as if no one but he understood the true nature of magic. You bloody fool. - He killed it. - It's OK. - He killed it. - He didn't. (..)2006 The Prestige