ثُمَّ إِنَّ عَلَيْنَا حِسَابَهُم
Sentence structure
ثُمَّ coordinates with delay, and إِنَّ opens the emphatic clause. عَلَيْنَا is the fronted predicate and حِسَابَهُم the delayed noun; attached هُم is a possessor.
Ayah summary
Then it is for Us to take a full account from them.
ثُمَّ — Then
Grammar: A conjunction marking sequence with delay.
إِنَّ — indeed
Grammar: An accusative/emphatic particle opening a nominal clause.
عَلَيْنَا — upon Us
Grammar: عَلَى is a preposition and attached نَا a first-person-plural genitive pronoun; the phrase is the fronted predicate of إِنَّ.
حِسَابَهُم — (is) their account
Grammar: حِسَابَ is the delayed accusative noun of إِنَّ; attached هُم is a third-person masculine-plural possessor.