An imperative directs an addressee to act, while a prohibition uses لَا with the negative jussive. Weak-final imperatives may lose their final radical and become strikingly short.
PerfectImperativeProhibition
وَجِيَاِيجَلَا تَوْجَ
وَلِيَلِلَا تَلِ
طَوَىاِطْوِلَا تَطْوِ
قَوِيَاِقْوَلَا تَقْوَ
أَغْنَىأَغْنِلَا تُغْنِ
اِلْتَقَىاِلْتَقِلَا تَلْتَقِ
سَمَّىسَمِّلَا تُسَمِّ
تَلَقَّىتَلَقَّلَا تَتَلَقَّ
The short لِ and longer اِطْوِ come from roots of the same three-radical length; the visible disparity comes from morphological deletions, not from shorter original roots.