A.T. Robertson Commentary Matthew 17:9

"And as they were coming down from the mountain, Jesus commanded them, saying, Tell the vision to no man, until the Son of man be risen from the dead." — Matthew 17:9 (ASV)

Until (εως ου). This conjunction is common with the subjunctive for a future event as his Resurrection (εγερθη) was. Again they were puzzled over his meaning. Jesus evidently hopes that this vision of Moses and Elijah and his own glory might stand them in good stead at his death.