In the process of grammaticalization, a lexical item changes into a grammatical one, or an already grammatical item acquires more grammatical properties. Efforts have been made to isolate the mechanisms and principles underlying or accompanying grammaticalization. In this article, such mechanisms and principles are thoroughly examined. It is worth noting that a concept subsumed under mechanisms by one scholar may be considered as a principle in another framework. To avoid this kind of confusion, here only the common principles and mechanisms accepted by most linguists are touched upon. Therefore, the principles presented in this article are based on the classic article of Hopper (1991) and the related mechanisms are those described by Wischer (2006). Examples, mostly from Persian language, are also provided for each principle or mechanism. A thorough understanding of these principles and mechanisms can help us to identify the instances of grammaticalization in language. This in turn gives us useful insights as to the nature of changes in language. The availability of texts belonging to different periods is of great help in identification of instances of grammaticalization. However, here we show that by a good understanding of (especially) the principles of grammaticalization, one can rather easily identify the instances of grammaticalization even in those languages without a written tradition.

Auxiliation is a subfield of grammaticalization which deals with the way the complex lexical verb structures, in some contexts, develope into auxiliary grammatical structures over time. Most auxiliaries in New Persian (Farsi) were used just as lexical verbs in Old or Middle Persian and converted into auxiliaries through grammaticalization. The present study deals with the process of auxiliation of three auxiliaries including "astidan", "bashidan" and "budan" (meaning "to be") which are used in Present Prepect, Past Subjunctive and Past Perfect structures in New Persian respectively; the question arises here is “ whether the usage of "astidan", "bashidan" and "budan" as auxiliaries in these structures goes back to the Old Persian or has been the outcome of grammaticalization” ? This paper tries to answer this question by scrutinizing instances of the aforementioned grammatical structures in Old and Middle Persian. According to existing evidence, the grammatical functions of these verbs as axiliray verbs in mentioned structures (Present Prepect, Past Subjunctive and Past Perfect) are due to the process of auxiliation in Middle Persian or Dari (early New Persian). The auxiliaries "astidan", "bashidan", "budan" don`t have a common root; the roots of mentioned auxiliaries are and meaning "to be" in Old Persian. Although in New Persian "ast" and "bash" are the present and imperative forms of "budan" respectively, "astidan" stems from while "budan" and "bashidan" stem from. The various forms of "astidan" which are used as lexical and linking verb as well as Present Prepect auxiliary in New Persian, didn't have auxiliary function in Middle or Old Persian. Although the auxiliation of this verb goes back to Middle Persian, its usage as Present Perfect auxiliary has been peculiar to New Persian or Dari. Infact "istadan" was substituded for "astidan" as Present Perfect axiliary in late Middle Persian or early Dari. "Bashidan" is used in various forms as lexical and linking verb as well as Past Subjunctive auxiliary in New Persian. But the auxiliary function of this verb has been the product of auxiliation in Dari. "Bashidan" had no usage in Old Persian and there has been found just one imperative form of it, "bash", as lexical and linking verb in Midde Persian. The auxiliary usage of "bashidan" in Past Subjunctive structure has been peculiar to New Persian or Dari. Past Subjunctive structures in Middle Persian texts had been formed by using "astidan" as auxiliary. Before being used as Present Perfect auxiliary in Dari, "astidan" was used as Past Subjunctive auxiliary in Middle Persian. Then "Bashidan" was substituded for "astidan" in Past Subjunctive structures in Dari. "Budan" not only is used as lexical and linking verb in various forms in New Persian, but also as an auxiliary verb in Past Perfect structures. The late function didn`t exist in Old Persian and "budan" was just a lexical and linking verb. The auxiliary "budan" is a grammatialized verb which was formed through auxiliation process in Middle Persian. Following a revolution in the manner of expressing the aspect of verb in Middle Persian, "budan" was used to express Perfect Aspect in Past Perfect structures. As mentioned, the auxiliaries "astidan", "bashidan" and "budan" as infinitives in New Persian stemmed from two different roots each of which passed through certain grammaticalization processes from Old Persian to Middle and New Persian, especially with regard to auxiliation. "Bashidan" went through this process later than two other auxiliaries and its auxiliary usage has been peculiar to Dari. "Astidan", "bashidan" and "budan" have been used as auxiliaries in grammatical structures of Present Prepect, Past Subjunctive and Past Perfect through reanalysis of predicative structures; they have lost their lexical category and meaning through "decategorialization" and "desemantization" process while their functions as lexical and linking verbs have been preserved. The later phenomenon is called "divergence" and is one of the grammaticalization principals which is common among many Indo-European languages. It is worth mentioning that conversion of existential verbs into auxiliaries in Present/Past Perfect structures have also been a very common tendency in grammaticalization process in many languages. According to Lehman (2002: 29) the existential "be" in English have gone through such auxiliation process. There have also been numerous evidences in different languages indicating linking verbs used as auxiliaries in structures with one Past Participle, such as Present Prepect, Past Subjunctive and Past Perfect structures. What we called resultative structure in this article seems very much like predicative structure in Persian. According to Bybee (1994: 69) evolution of resultative structures in to perfect structures can be an instance of "semantic generalization".