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.
(..) whoever they are, or however powerful they are. Least of all, my brothers, King Harald Finehair and his brother, for they are dedicated to our overthrow. Is that understood? We are all equally Ragnar's sons, but we are not equal in battle. So I will take the lead. I will establish the battle plans, (..)2017 TvShow: Vikings Title: Revenge Season: 4 Episode 18
(..) Then it's settled. What did you say to Margrethe? Uh... I told her that I've always loved her. But I understood you had first claim to her. Queen Lathgertha. Princess Ellisif from Denmark. Princess Ellisif. I'm surprised to see you. (..)2017 TvShow: Vikings Title: Revenge Season: 4 Episode 18