مَا جَاءَنِي زَيْدٌ لَكِنْ عَمْرٌو
Zayd did not come to me, but ʿAmr did.
«Lākin» conveys istidrāk, a corrective contrast: it prevents the hearer from drawing the wrong conclusion from what came before. It is particularly intelligible in a negative statement followed by affirmation of another referent.
«Lā», «bal» and «lākin» do not simply mean “and.” «Lā» excludes a second candidate; «bal» redirects the focus; «lākin» corrects an anticipated inference from the previous statement.