وَالْقَمَرِ إِذَا تَلَاهَا وَاللَّيْلِ إِذَا يَغْشَاهَا
By the moon when it follows the sun, and the night when it covers it. (91:2, 4)
Idhā attaches the oath objects to unfolding circumstances. The moon following and the night covering are not just labels for objects; they are scenes. The effect differs from simply saying “by the moon.”
Talāhā is morphologically past, while yaghshāhā is imperfect. The forms contribute to how events are portrayed, but tense labels alone cannot determine a unique rhetorical nuance independently of context.