Man: a conditional relative particle/noun. Tasahhara: the first past-tense verb within its condition. Wa-huwa yaẓunnu: a circumstantial clause explaining the mistaken belief. Anna al-fajra lam yaṭluʿ: the content of that belief.
Aw afṭara: coordinates the second case. Thumma tabayyana: introduces the later discovery. Qaḍā dhālika al-yawm: states the shared result. Lā kaffārata ʿalayhi negates an expiation, with lā of generic negation.