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.
(..) Why didn't you look for me? I sent you a note. I also searched for you at home. Varga laughed at me. But he is a good boy. He understood. He laughed. We broke up. He cares for you. You are afraid of everything. You are afraid. That's why you pretend to be brave. Because you are afraid. (..)1965 Forest of the Hanged
(..) Excuse me. A group of romanian prisoners was captured. We have the chance to hear from them the projector's position. The romanians refuse to talk. They say... I understood. Sir... Sir... They say that they don't know any foreign language. The General has made out of this a personal ambition. (..)1965 Forest of the Hanged