Optional Infinitive (OI) stage introduces a period in early language acquisition process through which finite and non-finite forms co-occur in similar contexts. With this regard, this book highlights the comprehension and production of OI features including application of infinitive markers, subject-verb agreement, and tense markers by Persian lower and upper intermediate EFL learners. The work aimed at exploring the effect of learners' proficiency level and L1 transfer on the acquisition of these features. Moreover, it attempted to apply its results to Truncation and Unique Checking Constraint as two major models proposed to explain OI features through a critical view. The results can provide second language teachers and syllabus designers with pedagogical feedback and helpful information on the related features attributed to each proficiency level.