Exegesis:

epērōtēsan de auton ‘they asked him.’ In view of vv. 12ff the subject is presumably the disciples.

didaskale ‘teacher,’ cf. on 3.12.

pote oun tauta estai ‘then when will these things be, or, happen?’ oun points back to v. 6 to which also tauta refers.

ti to sēmeion hotan mellē tauta ginesthai ‘what will be the sign when these things are about to happen?,’ with estai understood. The underlying idea is that events due to divine intervention are announced by a sign. To know the sign means to know that the events are about to happen. For sēmeion cf. on 2.12.

Translation:

They, or, ‘his disciples/followers.’

The sign when (or, that) this is about to take place, or more explicitly, ‘the sign which will show that this is about (or, that the time has come for this) to happen’ (cf. Good News Translation).

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.