Exegesis:

ei de kai ho Satanas eph’ heauton diemeristhē ‘and if Satan also is divided against himself.’ ei de kai implies that the general principle expressed in v. 17 may also apply to the realm of Satan. ho Satanas stands for ‘the kingdom of Satan’ as the rest of v. 18 shows.

pōs stathēsetai hē basileia autou ‘how will his kingdom stand firm?’ The clause implies a negative answer to the question.

hoti legete… ‘for you say…,’ and what they say implies that Satan is divided against himself. The clause introduced by hoti is only implicitly an explanatory clause.

Translation:

Satan … is divided against himself, or, ‘Satan is fighting against himself,’ ‘Satan fights Satan,’ ‘Satan’s subjects/followers/henchmen fight among themselves.’ The rendering should parallel v. 17. For Satan see on 10.18.

How will his kingdom stand, or, ‘hold out,’ ‘last,’ ‘continue to exist,’ ‘exist any longer’; or, if one has to shift from the noun to a verb, ‘how will he remain (or, go on) reigning?,’ ‘he certainly cannot rule any longer.’

For you say that I …, or, “—since, as you would have it, I…” (New English Bible), reminding Jesus’ interlocutors of their accusation (v. 15), which has been shown to be incompatible with facts everybody can know. This reminder, introduced by ‘for,’ states the reason for things not explicitly expressed in vv. 17f; hence some versions use a dash or dots (New English Bible, Bible de Jérusalem), others make the reason explicit, cf. e.g. ‘I spoke thus because you say that…’ (Kele, similarly Tzeltal, The Four Gospels – a New Translation), or shift to ‘but/yet you say…’ (Balinese, Sinhala).

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.