And this know, that if the goodman of the house... — Better, “if the master of the house.” See Notes on Matthew 24:43-44, where the words are almost identical.
Related Commentaries of Luke 12:39
Luke 12:39-40
39 And this know, that if the goodman of the house had known what hour the thief would come, he would have watched, and not have suffered his house to be broken through.
40 Be ye therefore ready also: for the Son of man cometh at an hour when ye think not.