A.T. Robertson Commentary Luke 21:6

"As for these things which ye behold, the days will come, in which there shall not be left here one stone upon another, that shall not be thrown down." — Luke 21:6 (ASV)

As for these things (ταυτα). Accusative of general reference.

One stone upon another (λιθος επ λιθω). Stone upon stone (locative). Here both Mr 13:2; Matthew 24:2 have επ λιθον (accusative). Instead of ουκ αφεθησετα (future passive) they both have ου μη αφεθη (double negative with aorist passive subjunctive). It was a shock to the disciples to hear this after the triumphal entry.