Exegesis:

egeneto nephelē kai epeskiazen autous ‘a cloud came and (for some time) overshadowed them.’ egeneto (aorist) refers to the punctiliar event of the appearing of the cloud, epeskiazen (imperfect) to the linear situation that followed. For episkiazō cf. on 1.35. autous may refer to all present, or to the three disciples only, preferably the former.

ephobēthēsan de en tō eiselthein autous eis tēn nephelēn ‘and they (i.e. the disciples) were frightened after they (i.e. Moses and Elijah) went into the cloud.’ ephobēthēsan describes the feeling of the three disciples at the end of the strange happening.

Translation:

Specifying the three pronouns of the third person plural in accordance with the interpretation preferred in Exegesis one may say. ‘all of them,’ ‘all (those who were) there/in that place’ for them, ‘the disciples for the first they, whereas the second they may be replaced by Moses and Elijah’ (Sranan Tongo), by ‘they two’ (Bahasa Indonesia KB), or by a deictic element indicating a group different from the group already referred to.

Came. ‘Clouds’ may not be said ‘to come’ but ‘to-be-there suddenly’ (Balinese), ‘to arrive unexpectedly’ (Bible de Jérusalem, survint), ‘to appear,’ ‘to emerge.’

Overshadowed, see on 1.35, of the two verbs mentioned there for Indonesian languages number (2) seems to be preferable here.

Entered the cloud, or, ‘disappeared into (or, were enveloped by) the cloud’ (Javanese), ‘the cloud encompassed/enveloped them’ (Tagalog).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.