«وإن كان الفعل مضارعًا منفيًا فالأمران جائزان»
If the imperfect verb is negative, both possibilities—using and omitting wāw—are permitted.
A negative imperfect reports the non-occurrence of a quality. Its difference from the affirmative form removes the particular restriction that made circumstantial wāw unsuitable. Yet a negative verbal clause can still directly describe its bearer's state. Hence either construction may be available when its context and syntactic connection support it.