لَا تُهْمِلُوا أَوْرَاقَهَا.
Do not neglect its leaves.
أَنْتُمْ لَا تُهْمِلُونَ أَوْرَاقَهَا.
You do not neglect its leaves.
In the first, “lā” prohibits and governs the jussive: “tuhmilū” has lost the indicative nūn. In the second, “lā” negates an indicative imperfect, “tuhmilūna,” whose nūn remains. The first is a prohibition; the second reports conduct.
When the intention is to correct behavior, the prohibitive form is apt. When one reports an existing practice, the indicative declarative form is apt. The difference is both grammatical and rhetorical.