In lam yakun, lam is the jussive/negative particle and yakun is a jussive imperfect of kāna. Qaddama is a Form II past-tense verb from q-d-m, meaning to bring forward or perform earlier.
Al-saʿya is the accusative object of qaddama. Ramala and saʿā are past-tense verbs functioning here in the conditional legal statement. Baʿdahu sets the temporal sequence: after it.