Aḥsin ilā Zaydin is an actual command to act kindly towards Zayd; aḥsin bi-Zaydin is an exclamation expressing admiration.
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A Real Command Versus an Exclamation
The Two Exclamatory Verbs: Forms, Rules and Parsing
Aḥsin ilā Zaydin is an actual command to act kindly towards Zayd; aḥsin bi-Zaydin is an exclamation expressing admiration.