Find the purpose phrase, then read the verb ending.
● جَعَلَ لَكُمُ اللَّيْلَ وَالنَّهَارَ لِتَسْكُنُوا فِيهِ وَلِتَبْتَغُوا مِنْ فَضْلِهِ (Al-Qasas 28:73)
The purpose phrases explain resting and seeking Allah's bounty; both second-group verbs lose the inflectional nūn.
● الَّذِي خَلَقَ الْمَوْتَ وَالْحَيَاةَ لِيَبْلُوَكُمْ أَيُّكُمْ أَحْسَنُ عَمَلًا (Al-Mulk 67:2)
لِيَبْلُوَكُمْ explains the stated purpose; the attached كُمْ is the object pronoun.