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.
(..) While you think about that... ...think about this: A revolver holds six bullets, not eight. I submit that this was not a hot-blooded crime of passion. That at least could be understood, if not condoned. No. This was revenge... ...of a much more brutal, cold-blooded nature. Consider this: (..)1994 The Shawshank Redemption