Exegesis:

etheōroun ton Satanan hōs astrapēn ek tou ouranou pesonta ‘I watched Satan fall like lightning from heaven.’ etheōroun is durative imperfect. For the background of this saying see commentaries. Satan’s fall from heaven means that he has lost his power, and this explains why the demons submitted to the disciples. hōs astrapēn ‘like lightning’ goes with pesonta. ouranos refers here to the abode of God.

theōreō ‘to see,’ ‘to watch.’

Satanas ‘Satan,’ ‘the devil.’

astrapē ‘lightning.’

Translation:

I saw Satan fall, or better to bring out the durative aspect, “I watched how Satan fell” (New English Bible), “I was watching and saw Satan fall” (Phillips). Satan is usually transliterated, but the name may also be translated by the term for “devil” (for which see on 4.2a); the two designations do not occur in the same passage. In Muslim countries the Arabic form of the name, Shaitan, usually is well known, but it may have acquired a different meaning, e.g. in Bahasa Indonesia, Javanese, where it is a designation of ‘demons’ in general.

Like lightning, or to make clear what the comparison refers to, ‘like lightning falls, or, goes down,’ or a more specific verb like ‘strikes.’ Batak Toba possesses distinctive terms for a ‘flash-of-lightning (as seen in the sky),’ and ‘stroke-of-lightning (coming down to the earth),’ used here.

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.