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.
(..) everywhere. (whooshing sound) See, the priorities are there. ''Money for the children.'' They heard that, understood it and responded. Anything else is (irritably) ''money for the children.'' You have to say it like that. (irritably) ''Come here, come here, come here.'' My father began to pass (..)1983 Bill Cosby: Himself