دَخَلَ زَيْدٌ يَبْتَسِمُ.
Zayd entered smiling.
دَخَلَ زَيْدٌ وَهُوَ يَبْتَسِمُ.
Zayd entered while he was smiling.
In the first, the circumstantial clause begins directly with the affirmative imperfect “yabtasimu”, without a separate circumstantial wāw. In the second, “huwa” serves as a nominal subject and “yabtasimu” as its predicate, creating a nominal circumstantial clause introduced by wāw.
The wāw in the second version should not be treated as part of the imperfect verb. Both expressions describe Zayd's state on entry, but the internal clause structure and linking mechanism differ.