هَلْ تَشْكُرُونَ؟
Will you give thanks?
“Hal tashkurūn?” asks about a future occurrence of the act of giving thanks. “Fa-hal antum shākirūn?” presents gratefulness as a predicate or quality and can strongly foreground its realization and steadiness.
The issue is not that only one sentence is grammatical; the choice is rhetorical. The speaker may focus either on a fresh act of thanks or on the addressees as people disposed to gratitude.
ExpressionRhetorical angleFocus
Hal tashkurūn?Imperfect verbOccurrence/renewal of thanks
Fa-hal antum shākirūn?Nominal predicateState or quality of being grateful
Fa-hal antum tashkurūn?Verb after pronounDisputed/objectionable here