The textbook example: denial calls for emphasis

Fasaha of the Speaker, Balagha of Discourse and Contextual Appropriateness

كون المخاطب منكرا للحكم حال تقتضي تأكيد الحكم
The addressee's denial of a proposition is a situation that calls for emphasizing the proposition.

إِنَّ زَيْدًا فِي الدَّارِ
Indeed, Zayd is in the house.

Here the denial of the hearer motivates the confirmatory particle inna. Zaydan is the accusative noun governed by inna; fi al-dar is the prepositional phrase functioning as its predicate.

The lesson is not to insert inna into every statement. It is to understand why denial can make emphasis appropriate.