A.T. Robertson Commentary Luke 12:39

"But know this, that if the master of the house had known in what hour the thief was coming, he would have watched, and not have left his house to be broken through." — Luke 12:39 (ASV)

The thief (ο κλεπτης). The change here almost makes a new parable to illustrate the other, the parable of the housebreaking (verses 39,40) to illustrate the parable of the waiting servants (35-38). This same language appears in Mt 24:43f. "The Master returning from a wedding is replaced by a thief whose study it is to come to the house he means to plunder at an unexpected time" (Bruce). The parallel in Mt 24:43-51 with Lu 12:39-46 does not have the interruption by Peter.

He would have watched (εγρηγορησεν αν). Apodosis of second-class condition, determined as unfulfilled, made plain by use of αν with aorist indicative which is not repeated with ουκ αφηκεν (first aorist active indicative of αφιημ, κ aorist), though it is sometimes repeated (Matthew 24:43).