جَزَآءً مِّن رَّبِّكَ عَطَآءً حِسَابًا
Sentence structure
جَزَآءً is an accusative verbal noun dependent on an understood verb: they are rewarded as a reward from your Lord. عَطَآءً restates that reward, and حِسَابًا describes the gift as ample and sufficient.
جَزَآءًۭ — as a reward
Grammar: An indefinite masculine accusative verbal noun functioning as a cognate object of an understood verb.
مِّن — from
Grammar: A preposition beginning the phrase that identifies the source of the reward.
رَّبِّكَ — your Lord
Grammar: رَبِّ is genitive after مِّن. The attached كَ is a second-person masculine singular possessive pronoun in the genitive position.
عَطَآءً — a gift
Grammar: An indefinite masculine accusative noun in apposition to جَزَآءً.
حِسَابًۭا — ample and sufficient
Grammar: An indefinite masculine accusative adjective describing عَطَآءً; morphologically it is a form III verbal noun used adjectivally.