then, thereupon: the narrative word that marks what happens next, moving a story from one event to the following one.
This is the workhorse connector of the Aramaic stories in Daniel and Ezra. It sits at the head of a sentence and tells the reader that the action is moving on: the king gives an order, and then the wise men are summoned; the music plays, and then everyone bows down. It keeps a fast-moving account marching forward one step at a time.
It does double duty as "at that time." Sometimes it simply chains the next event (thereupon, next), and sometimes it points to the particular moment when something happened (at that time, right then). English "then" covers both, so the word rarely trips up a reader.
It shows up in two shapes that mean the same thing. One is the bare word, and the other adds a small prefix meaning "in" or "at," giving the sense "at that point." The two are used interchangeably in the same books, often within a few verses of each other, and translations render both simply as "then."