Exegesis:

blepete oun pōs akouete ‘take care, then, how you listen.’ oun characterizes the clause as an inference from the preceding. For blepō meaning (to take care).

hos an gar echē, dothēsetai autō ‘for (he) who has, to him will be given,’ referring either to faith (pisteusantes, v. 12; pisteuousin, v. 13), or, to the bearing of fruit (karpophorousin, v. 15), preferably the former. The agent of dothēsetai is God.

hos an mē echē, kai ho dokei echein arthēsetai ap’ autou ‘(he) who has not, even what he thinks he has will be taken from him.’ For airō cf. on 6.29f.

dokeō ‘to believe,’ ‘to think’ (of subjective opinion), ‘to seem.’

Translation:

Take heed then how you hear, or, ‘look to the manner you hear the word’ (Sranan Tongo). For take heed.

To him who has will more be given, or, ‘he who has (some), he shall receive (or, to him God shall give) more (than that).’ Comparable changes will then be necessary in the next clause, using e.g. ‘lose’ or ‘forfeit’ (cf. Phillips, New English Bible), as the opposite of ‘receive.’

What he thinks he has, avoiding the paradoxical wording Mark has. Other possible renderings are, ‘the things he supposed to be his’ (Bahasa Indonesia), ‘even what he called (or, said to be) his possessions’ (cf. Toraja-Sa’dan). For to think cf. on “to suppose” in 2.44.

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.