إِنَّ الْإِنْسَانَ لِرَبِّهِ لَكَنُودٌ
Indeed the human being is deeply ungrateful to his Lord. (100:6)
Inna is an emphatic particle; the lām of la-kanūd reinforces the predicate further. After the oath, the combined emphasis signals the gravity of the assertion. It does not prove that the audience is necessarily an active denier; significance, inattention or urgent warning can also call for strong reinforcement.
ElementRhetorical contribution
OathPrepares attention for a weighty assertion
InnaConfirms the predicative relation
Predicate lāmAdds further assurance and force