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.
(..) how Loki used it to turn two of the sharpest men I know into his personal flying monkeys. Monkeys? I do not understand. I do. I understood that reference. Shall we play, Doctor? This way, sir. The gamma readings are definitely consistent with Selvig's reports of the Tesseract. (..)2012 The Avengers