The present study sought to investigate the acquisition of referential, quasi and expletive subject pronouns, three different types of obligatory subjects in English, by adult Persian speaking L2 learners of English at different stages of L2 acquisition. A Grammaticality Judgment Test and a Translation Test were designed and developed to elicit the participants' knowledge of obligatory subjects in English. The results revealed that the L2 learners conformed to the natives in their performance on obligatory referential subjects and obligatory expletive subjects, but they did not conform to the natives in the case of obligatory quasi subjects. The obtained results can be accounted for by the fact that Persian as the L2 learners’ native language accepts referential and expletive subjects in certain structures, but no quasi subjects exist in Persian. This lends support to Smith and Tsimpli (1995) and Hawkins and Chan (1997) in that adult second language learners are not able to modify those functional features not already encoded in the entries of their native language.