This study explored the syntactic acquisition by Persian monolingual and Kurdish-Persian bilingual learners of English as a second and third language and the effect of instruction on beginner learners' syntactic acquisition with reference to UG-based non-primary language learning theories. A group of EFL learners (23 Persian monolingual and 73 Kurdish-Persian bilingual learners) were selected. They were divided into beginner, intermediate and advanced levels based on their performance on the Oxford Placement Test. A grammaticality judgment test, a translation test and a functional test were used to collect data. ANOVA analyses indicated that Kurdish- Persian bilingual learners of L3 English and Persian monolingual learners of L2 English were able to reset the parametric variations related to the intended features at intermediate and advanced stages of L3 acquisition, but they failed to do so at the initial stage. The Direct Access (DA) hypothesis, the Full Transfer/Full Access (FTFA) hypothesis and the Modulated Structure Building Hypothesis (MSBH) were found to best explain the findings. Paired-samples t -test showed that instruction positively influenced the acquisition of the syntactic features by beginner-level learners after the treatment.
