Yuṣallī is an imperfect verb and al-imāmu its nominative subject. Bi-al-nāsi is a prepositional phrase. Al-maghriba wa-al-ʿishāʾa name the two prayers here in the accusative.
Bi-adhānin wa-iqāmatin describes how the prayers are conducted. Both nouns are singular, supporting one call of each kind.