فَوَسَطْنَ بِهِۦ جَمْعًا ٥
Sentence structure
فَوَسَطْنَ is a coordinated feminine-plural perfect with attached subject نَ. بِهِۦ links to it, and جَمْعًا is an accusative circumstance.
Ayah summary
Then they penetrate into the midst of the enemy ranks.
فَوَسَطْنَ — then penetrate the midst
Grammar: فَ coordinates; a feminine-plural perfect with attached نَ as subject.
بِهِۦ — thereby
Grammar: A preposition with a masculine-singular genitive pronoun, linked to وَسَطْنَ.
جَمْعًا — a host
Grammar: An indefinite masculine accusative circumstance.