
The study explores the extent to which the intrinsic complexity of relative clauses (RCs) and prior linguistic knowledge impact the acquisition of RCs by L2 learners. The study investigates the main sources of the erroneous and avoided types of English RCs produced by Persian-speaking learners of English at three proficiency levels. The data elicitation task was a translation test comprised of six types of RCs modeled on the RC types in the Noun Phrase Accessibility Hierarchy. To analyze the data, the occurrence frequencies of the correctly and erroneously formed RCs were counted and the avoided RCs were identified in each RC type. Then, a precise error analysis was done. The statistical analysis of 3840 RCs showed that the most common error types were (i) forming English RCs with resumptive pronouns and (ii) altering more-marked RCs with non-canonical word order to less-marked RCs with canonical word order. The errors are interpreted as evidence for the impact of both L1 transfer and the universal intrinsic constraints of RCs. The analysis of the avoided RC types, mostly the more marked RCs, indicates that avoidance is mainly linked to the universal intrinsic constraints of RCs.