«فإن كانت فعلية والفعل مضارع مثبت امتنع دخولها»
If the circumstantial clause is verbal and its verb is affirmative imperfect, circumstantial wāw cannot enter it under the ordinary rule.
“Affirmative” here means an imperfect verb whose proposition is not negated. The verbal ḥāl is connected to its bearer through a pronoun or another suitable link. Importantly, the prohibition concerns circumstantial wāw in this construction; it is not a claim that every wāw preceding an imperfect is impossible.