نَحْوَ قَوْلِهِ تَعَالَى: ﴿وَلَعَبْدٌ مُؤْمِنٌ خَيْرٌ مِنْ مُشْرِكٍ﴾.
Translation: “A believing bondman is indeed better than a polytheist” (Qurʾān 2:221, excerpt).
«عَبْدٌ» is indefinite but qualified by «مُؤْمِنٌ»; this permits it to serve as the mubtadaʾ. «خَيْرٌ» is the khabar. The point is qualification, not a blanket license for every bare indefinite to head a statement.