إِذَا جَاءَ نَصْرُ اللَّهِ وَالْفَتْحُ
When Allah's help and victory come. (110:1)
Idhā is a temporal adverb carrying a conditional sense. Rather than immediately state a command, the verse first identifies the situation in which that command becomes relevant. Jāʾa is morphologically past, yet the construction with idhā may concern a forthcoming occurrence.
The rhetorical effect is to present the event as a concrete, intelligible condition. The past form alone does not provide a historical date; the responsive command emerges later at fa-sabbiḥ.