What is the correct grammatical difference between “lā tuhmilū” and “lā tuhmilūna”?

Supplementary Practice 11: Report, Objection-Reply and Instruction

• The first is a report and the second a command
• Both are past-tense verbs
• The first drops nūn under prohibitive lā; the second retains it under negative lā
• Both retain nūn in the same way