إِنَّهُۥ كَانَ فِىٓ أَهْلِهِۦ مَسْرُورًا
Sentence structure
Attached هُۥ is the noun of إِنَّ, and كَانَ فِىٓ أَهْلِهِۦ مَسْرُورًا is its predicate clause. كَانَ has understood هُوَ as its noun and مَسْرُورًا as its accusative predicate.
Ayah summary
In the world, that person lived happily with the family while heedless of the Hereafter.
إِنَّهُۥ — Indeed, he
Grammar: إِنَّ is accusative and emphatic; attached هُۥ is its accusative noun.
كَانَ — had been
Grammar: A deficient third-person masculine-singular perfect with understood هُوَ as its noun and مَسْرُورًا as its predicate.
فِىٓ — among
Grammar: A preposition; the phrase فِىٓ أَهْلِهِۦ is linked to كَانَ.
أَهْلِهِۦ — his people
Grammar: أَهْلِ is genitive after فِىٓ and attached هِۦ its genitive possessor.
مَسْرُورًا — happy
Grammar: An indefinite accusative passive participle functioning as the predicate of كَانَ.