فِيهَا عَيْنٌۭ جَارِيَةٌۭ
Sentence structure
فِيهَا is the fronted predicate and عَيْنٌ the delayed topic. جَارِيَةٌ is its nominative adjective.
Ayah summary
A spring will be flowing in that garden.
فِيهَا — Therein
Grammar: A prepositional phrase serving as the fronted predicate; attached هَا is genitive.
عَيْنٌۭ — (will be) a spring
Grammar: An indefinite feminine-singular nominative delayed topic.
جَارِيَةٌۭ — flowing
Grammar: An indefinite feminine-singular nominative active-participle adjective of عَيْنٌ.