جَاءَ التِّلْمِيذُ لَا يَتَكَلَّمُ.
The pupil came without speaking.
جَاءَ التِّلْمِيذُ وَهُوَ لَا يَتَكَلَّمُ.
The pupil came while he was not speaking.
Both may convey silence during the entrance, but the second is formally a nominal clause with a stated subject pronoun. The analysis of a negated imperfect also depends on the negating particle and construction; one must not extrapolate the bare affirmative imperfect rule indiscriminately to every negative or pronominal clause.
Identify the precise clause type and the device linking the circumstance to its bearer before asserting what wāw may or may not do.