Middle Persian past sentences are constructed ergatively, i. e. transitive objects and intransitive subjects are coded by the auxiliaries, while transitive subjects are left uncoded.In this paper some characteristics of this ergative construction are presented. These are:(1) there is a split conditioned by tense (aspect) of sentence, (2) the distributional pattern of cases, where they exist, conforms to the ergative pattern, (3) the distribution of subject properties in the present is different from that in the past, and(4) there is a difference in relative clause formation between present and past tenses.