Law may introduce a negative verb phrase or another negated proposition. One must identify the scope of negation: is nonperformance itself the supposed condition, or is the consequence negated?
Example: «لو لم يجتهد لفشل» — If he had not worked hard, he would have failed. Here not working hard is the condition and failure is the consequence. Reversing those roles changes the statement.
When law interacts with ma, lam or la, the text distinguishes negation of an event from possible negation of its duration. Interpret the whole predication rather than one particle in isolation.