Wei (為) had already become a copula in Archaic Chinese which can introduce both nominal and verbal components functioning as the predicate, provided the latter were referential in predicate position. Since the Medieval ages, especially in the Chinese-translated Buddhist scriptures, functional expansion occurred regarding wei due to a large number of translation of be verb in the original Sanskrit texts. Wei can not only introduce verbal predicates, but also help to form the passive voice and interrogative sentence and to strengthen the mood. Thus, wei exhibited the distinctive and complete properties of an auxiliary verb, which laid an important syntactic and semantic foundation for further evolution to a modal adverb and an alternative conjunction.
