This study examines the agreement of verbs with subjects in terms of person and number in the writing sheets of Azerbaijani-Persian bilingual students. The data were collected from compositions of students in 4th and 5th grades of primary school. Errors related to verb usage in the writing of both groups of students were extracted, and after determining the frequency and percentage of these errors, they were analyzed based on various factors involved in L2. The findings of this research showed that students in both the 4th and 5th grades made various linguistic errors in using the person and number inflectional morphemes of verbs, while they did not face issues in using other inflectional morphemes of verbs such as negation, tense, voice, and aspect. The disparity in the occurrence of errors related to inflectional morphemes of verbs led data analysis towards examining the differences in verb inflectional morphemes and classifying them. The person and number morphemes of verbs function at the sentence level to ensure agreement between verb and subject, whereas other inflectional morphemes of verbs are considered inherent morphemes that perform a morphological function. The purpose of attaching inherent morphemes to verb is to convey meaning related to time, negation, and aspect to the audience. Therefore, learning inherent inflectional morphemes, which carry more semantic weight, occurs much faster and more easily than learning those morphemes that have a syntactic function, such as ensuring subject-verb agreement. Additionally, the higher frequency of errors in use of person and number morphemes in 4th-grade students compared to 5th-grade students indicates that advancing to the next grade significantly impacts second language learning. Thus, the results of this study highlight the role of both linguistic and non-linguistic factors in L2, which can be applied in various fields of language education (such as curriculum development and teaching).