1. Introduction

 Mamasani Lori dialect is one of the southern dialects of Lori language and a branch of southwestern Iranian dialects which, like many other Iranian dialects, has significant phonetic and phonological variations. One of the notable morphonological domains in this dialect is the representation of various forms of the past morpheme. The main issue of this research is how to optimize the processes, constraints and rankings of these morphonological variations based on the optimality theory, and investigating whether there are universal rules for them? The importance and necessity of the current study is to preserve the heritage of the ancient Iranian languages and to provide a comprehensive description of one of the morpho-phonological sub-fields of the studied dialect. In this regard, it has been attempted to answer the following research questions while analyzing the subject: 1) What are the allomorphs of the past morpheme in Lori Mamasani's dialect?  2) In the framework of optimality theory, what processes, constraints, and rankings govern the optimal output of the allomorphs of the past morpheme, and are they governed by universal rules? 

2. Methodology

 The method of the present research is descriptive-analytical and the method of data collection is documentary, based on the list of simple verbs of Persian language (Tabatabai, 1376), a list of equivalent verbs in Mamasani Lori dialect was extracted and transcribed based on the International Phonetic Alphabet and all contexts of past allomorphs are analyzed in the data. 

3. Findings

 The findings showed that the tendency to weakening in the dialect of Lori Mamasani is a very common and widespread process. Data analysis showed that /ɪd/ is the underlying past morpheme in this dialect, based on the criteria of context distribution, frequency of occurrence and application of the least rules. The past morpheme includes the nine allomorphs (/ Ið /, / ð /, / ɑð /, / d /, / t /, / es /, / s /, / ɑs / and / Ø) which are realized as a result of weakening process in this dialect. In this study, some degrees of weakening in the consonant and vowel of the past morpheme / ɪd / was observed, which indicates that in general the weakening process is a function of the global constraints of LAZY in this dialect. Also, a weakening continuum has been observed in the vowel of these allomorphs, in which the tense high vowel / ɪ / in the allomorph / ɪð / is weakened to the central vowel / e / in the allomorph / es / and the lax low vowel / ɑ / in the allomorphs / ɑs / and / ɑ /. The * V (tense) constraint governs this process. In more than half of the examined data (246 verbs out of the total 446 verbs), the past morpheme realized in two forms, / Ið /, / es /. The allomorph / es / is  realized as a result of the devoicing process of the voiced dental fricative phoneme / ð / in the allomorph /Ið/,  which immediately realized as /s/,that is  its  closest phoneme based on the place of articulation feature, because there is no voiceless dental fricative  phoneme / ɵ / in the phonological system of this dialect. 

4. Discussion and Conclusion

 The process of consonantal devoicing under the influence of sociological factors is a common process in Iranian languages ​​and dialects. In this study, the importance of consonant and vowel weakening processes in selecting optimal outputs and allomorphs of the past morpheme is shown. It seems that this study can be generalized to the allomorphs of the past morpheme in Persian as well, because in terms of the criteria occurrence frequency, context/texture and application of the least rules in Persian, the allomorph / ɪd / can be considered as the underlying past morpheme in Persian, Contrary to the opinion of Karampour et al.(1390) who believed that other allomorphs are realized as a result of phonological processes and derivational stages/steps in Serialism optimality which has generated the other allomorphs in surface realization. Perhaps in this way it may be possible to put aside the classical distinctions and definitions of the past-stem and the present–stem and believe only in the present-stem or verb-stem, which can be redefined by adding the past morpheme / ɪd / and applying processes such as elision, insertion, assimilation, etc. based on different phonological contexts and intermediate derivational stages in optimality approach. The present study also confirms the superiority of the performance and mechanism of the Serialism to the Parallelism (standard) optimality, because, as noted, the allomorphic variations of the past morpheme are the result of successive processes of the Serialism sequential steps, in which, at each stage, a process or change is applied to the input, and a series of feeding or bleeding processes are involved in the selection of the final output, which explains the course of evolution in the intermediate stages. In parallelism, the processes and constraints of the intermediate stages of derivation are not dealt with, and the surface forms show different and varied phonetic realizations, which are classified as irregular categories in traditional grammars..