فَإِذَا فَرَغْتَ فَانْصَبْ
So when you have finished, then devote yourself to effort. (الشرح: 7)
Idhā introduces a conditional temporal frame, faraghta expresses the condition, and fa-nṣab gives the imperative response with fā. There is an unmistakable shift from report to directive: the recalled blessings and assurance of ease are followed by a demand for action.
The imperative invites renewed purposeful exertion once the preceding engagement is complete. This condition–response relation is not the same as merely using wāw to coordinate two independent reports. The sequence itself determines how the directive should be understood.