عَادَ الْعُمَّالُ وَلَمْ يُكْمِلُوا الْعَمَلَ.
The workers returned without having completed the work.
Read circumstantially, wa-lam yukmilū describes the workers' condition on returning: the task remained unfinished. Lam negates the imperfect with past-time force. The clause need not narrate another action taking place after they returned.
In a different narrative, the same words could be taken as two coordinate assertions. Circumstantial versus coordinating wāw therefore depends on temporal relation, the referent of the state, and the actual communicative purpose—not on the presence of lam alone.