الْحَجُّ وَاجِبٌ عَلَى الْأَحْرَارِ
Translation: Hajj is obligatory upon free persons.
Al-ḥajju is a nominative subject; wājibun is its nominative predicate; ʿalā al-aḥrār is a prepositional phrase linked to “obligatory.” The later descriptors further qualify the persons mentioned.