How did the past tense systems of Iranian languages change during the Middle Iranian period?

During the Middle Iranian period, the complex Indo-European verbal system underwent significant transformation, specifically regarding the past tense. The inherited past tense system was largely abandoned, leaving behind only a few relics, such as the partial survival of the augment in the Sogdian imperfect tense.1

In its place, a new past tense system emerged. This development was based on the old past participle, which was frequently combined with auxiliary verbs to express past actions. This shift also introduced distinctions between transitive and intransitive verbs within the past tense system. In certain languages, such as Khotanese and Pashto, this new system further incorporated distinctions of gender and number.1