This study set out to investigate the distribution of the syntactic properties associated with the prodrop parameter by Persian-speaking L2 learners of English in an attempt to examine the (un)attainability of native-like knowledge and (in)accessibility of UG in adult SLA. To do so, 92 participants were assigned into different levels of L2 knowledge based on their performances on the Oxford Placement Test (2001). A grammaticality judgment test and a translation test were developed and validated in this study. Analysis of the results indicated the following hierarchy of difficulty: referential subjects > expletives > verb-subject constraint > quasi subjects > thattrace constraint. Findings evidence that native-like attainment of unselected L2 syntactic features is unlikely at advanced stages, but it is logically possible that Persian-speaking L2 learners of English attain native-like knowledge of [-prodrop] at very advanced or near-native stages of L2 acquisition.