Not all denials are equally strong. A mild rejection may be answered with one emphatic device; firm resistance may call for an oath or multiple reinforcements.
Mild denial: “إِنَّ زَيْدًا حَاضِرٌ” — inna provides emphasis.
Stronger denial: “إِنَّ زَيْدًا لَحَاضِرٌ” — inna plus emphatic lām.
Intense denial: “وَاللَّهِ إِنَّ زَيْدًا لَحَاضِرٌ” — an oath adds further strength.
These are worked applications. The principle is to maintain a suitable proportion between the intensity of denial and the degree of emphasis.