احْفَظُوا الْكُتُبَ.
Protect the books.
أَنْتُمْ تَحْفَظُونَ الْكُتُبَ.
You protect the books.
The first is a directive speech act, requesting conduct. The second is a declarative assertion that can be assessed as true or false. Guidance for students and a report of accomplished conduct call for different forms.
The social force of an imperative may vary with context: command, advice or request. Its morphology alone does not establish the precise interpersonal force in every situation.