In Vedic literature, mitrá- denotes, first, “alliance; covenant” as a neuter common noun; second, an “ally” who participates in this alliance as a masculine noun; and finally, the god Mitra, who bears this name, as a masculine proper noun. As a neuter nominative/accusative form, mitrám appears in the fixed expression mitrám as/bhū. In this expression, the subject can be either a masculine or feminine substantive, but the predicate noun is always mitrám. There are differences in gender between the subject and mitrám. Moreover, from a semantic perspective, it is quite unlikely that this expression means “the subject is the （abstract） alliance itself.” Therefore, this expression can be a special expression that is not an ordinary nominal predicate sentence. For this study, a survey was conducted of the Vedic literature, yielding a total of 15 examples of mitrám as/bhū. The parallelism of examples with the usual nominal predicate mitráḥ as/bhū shows practically no difference in meaning between mitrám as/bhū and mitráḥ as/bhū. In conclusion, mitrám as/bhū should be regarded as a so-called idám bhū construction and structurally it is most appropriate to understand mitrám as the accusative of content （“Inhaltsakksativ”）. The transitive expression of mitrám as/bhū as an idám bhū construction also appeared, which helps us clarify the structure of the idám bhū construction.
