A.T. Robertson Commentary Luke 9:36

"And when the voice came, Jesus was found alone. And they held their peace, and told no man in those days any of the things which they had seen." — Luke 9:36 (ASV)

When the voice came (εν το γενεσθα την φωνην). Another example of Luke's idiom, this time with the second aorist middle infinitive. Literally, "on the coming as to the voice" (accusative of general reference). It does not mean that it was "after" the voice was past that Jesus was found alone, but simultaneously with it (ingressive aorist tense).

Alone (μονος). Same adjective in Mr 9:8; Matthew 17:8 translated "only." Should be rendered "alone" there also.

They held their peace (εσιγησαν). Ingressive aorist active of common verb σιγαω, became silent. In Mr 9:9; Matthew 17:9, Jesus commanded them not to tell till His Resurrection from the dead. Luke notes that they in awe obeyed that command and it turns out that they finally forgot the lesson of this night's great experience. By and by they will be able to tell them, but not "in those days."

Which they had seen (ων εωρακαν). Attraction of the relative α into the case of the unexpressed antecedent τουτων. Perfect active indicative εωρακαν with Koine (papyri) form for the ancient εωρακασιν changed by analogy to the first aorist ending in -αν instead of - ασιν.