«إِبْرَازُ غَيْرِ الْحَاصِلِ فِي مَعْرِضِ الْحَاصِلِ»
Presenting what has not yet occurred in the manner of something already realized.
A past form can do more than shift time reference. It may make future occurrence appear certain or vividly established, as if its causes already assured it. The listener is invited to regard an expected event as practically accomplished.
This does not justify one mechanical explanation for every passage. Firm causes, certainty, strong desire and other situational motives must be evaluated separately.