لَا تَكْذِبْ
Do not lie.
Syntax: lā is prohibitive and jussive; “takdhib” is an imperfect in the jussive marked by sukūn; its subject is the implied pronoun “anta.”
لَا يَكْذِبُ الرَّجُلُ
The man does not lie.
Syntax: lā negates; “yakdhibu” remains indicative, marked by ḍamma; “al-rajulu” is the nominative subject.
The same written particle can have different grammatical functions. Governance, verb inflection, and communicative intention determine the analysis.