إِنَّ بَطْشَ رَبِّكَ لَشَدِيدٌ
Sentence structure
بَطْشَ is the accusative noun of إِنَّ and is annexed to رَبِّكَ. In لَشَدِيدٌ, لَ is emphatic and شَدِيدٌ the nominative predicate.
Ayah summary
Your Lord’s seizure of oppressors and criminals is exceedingly severe.
إِنَّ — Indeed
Grammar: An accusative and emphatic particle; its noun and predicate follow.
بَطْشَ — (the) Grip
Grammar: An accusative verbal noun functioning as the noun of إِنَّ and annexed to رَبِّكَ.
رَبِّكَ — (of) your Lord
Grammar: رَبِّ is the genitive complement of بَطْشَ and attached كَ its genitive possessor.
لَشَدِيدٌ — (is) surely strong
Grammar: لَ is the emphatic lām; شَدِيدٌ is the indefinite nominative predicate of إِنَّ.