لَّسْتَ عَلَيْهِم بِمُصَيْطِرٍ
Sentence structure
لَّسْتَ is a defective negative verb and attached تَ its noun. بِمُصَيْطِرٍ is its predicate, with an extra emphatic bāʾ, and عَلَيْهِم is linked to that predicate.
Ayah summary
You are not an overseer imposed upon people to force them to believe.
لَّسْتَ — You are not
Grammar: لَسْ is a defective negative perfect; attached تَ is its second-person masculine-singular noun.
عَلَيْهِم — over them
Grammar: عَلَى is a preposition and attached هِم a third-person masculine-plural genitive pronoun; the phrase is linked to the predicate. This phrase is linked to «مُصَيْطِرٍ».
بِمُصَيْطِرٍ — a controller
Grammar: بِ is an extra emphatic preposition; مُصَيْطِرٍ is a genitive-in-form active participle functioning as the predicate of لَيْسَ in the accusative position.