Exegesis:

ou gar estin dendron kalon poioun karpon sapron ‘for there is no good tree which bears rotten fruit,’ or, ‘a tree is not good when it bears rotten fruit,’ taking kalon as predicate. The former is preferable. gar expresses here continuation, not explanation.

sapros ‘rotten’ (here of rotten fruit and a rotten tree), or, somewhat more general, ‘bad.’

oude palin ‘nor, on the other hand.’ Several translations do not render palin (cf. e.g. Translator’s New Testament, An American Translation).

palin ‘again,’ here ‘in turn,’ ‘on the other hand.’

Translation:

Another possible change in the structure is, ‘a good tree does not bear bad fruit,’ etc. (cf. Good News Translation).

For good or bad trees bearing bad or good fruit.

To bear fruit (for which cf. 3.8) is used here in a literal sense, but the sentence as a whole has figurative meaning.

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.