Negative particles can differ in force, time reference and effect on the imperfect. Prohibitive lā forbids and produces the jussive; lam negates a past event and produces the jussive; lan negates a future event and produces the subjunctive.
Word / formMeaning / explanation
لَا تَضْرِبْdo not strike.
لَمْ تَضْرِبْyou did not strike.
لَنْ تَضْرِبَyou will not strike.
All three are negative in some sense, but their grammatical operation and sentence function differ. This distinction also appears in the source exercises.