فَلَآ أُقْسِمُ بِٱلْخُنَّسِ
Sentence structure
فَ resumes the discourse; the exact grammatical explanation of لَا has recognized differences. أُقْسِمُ is first-person Form IV imperfect, and بِٱلْخُنَّسِ is the oath phrase.
Ayah summary
An oath is introduced by entities described as retreating; the next ayah continues their description.
فَلَآ — But nay
Grammar: فَ resumes the discourse. The exact grammatical status of لَا has recognized differences.
أُقْسِمُ — I swear
Grammar: A Form IV active imperfect, first-person singular: I swear.
بِٱلْخُنَّسِ — by the retreating ones
Grammar: بِ is the oath preposition; the definite plural noun is genitive.