A.T. Robertson Commentary Luke 9:34

"And while he said these things, there came a cloud, and overshadowed them: and they feared as they entered into the cloud." — Luke 9:34 (ASV)

Overshadowed them (επεσκιαζεν αυτους). Imperfect active as present participle in Mr 9:7, inchoative, the shadow began to come upon them. On Hermon as on many high mountains a cloud will swiftly cover the cap. I have seen this very thing at Blue Ridge, North Carolina. This same verb is used of the Holy Spirit upon Mary (Luke 1:35). Nowhere else in the N.T., though an old verb (επι, σκιαζω, from σκια, shadow).

As they entered into the cloud (εν τω εισελθειν αυτους εις την νεφελην). Luke's idiom of εν with the articular infinitive again (aorist active this time, on the entering in as to them). All six "entered into" the cloud, but only Peter, James, and John "became afraid" (εφοβηθησαν, ingressive first aorist passive).