لَا تَشْتِمْنِي يَكُنْ خَيْرًا لَكَ
Do not insult me; that will be better for you.
The first jussive, tashtimnī, is governed by prohibitive lā. The following yakun is jussive as the consequence of the request. Their jussive moods have distinct grammatical explanations.
The understood condition is “if you refrain from insulting me, it will be better for you,” not “if you insult me.” Preserving the negative content of the prohibition is essential to analyzing the consequence.
VerbCause of jussiveContribution
TashtimnīProhibitive lāRefrain from insulting
YakunResponse to requestThe resulting benefit