Articles in general and definite articles in particular can create problems even long after all other aspects of English have been mastered. The present article investigated the learnability problems related to the acquisition of count-mass distinction of English nominals by Persian L2 learners. The theoretical underpinning of the study is the interpretability hypothesis (Tsimpli & Dimitrakopoulou, 2007) arguing that the features which are semantically interpretable can be acquired. To this end, 50 learners constituted the participants of the study and completed a forced-choice elicitation task requiring the use of articles. The results of the study substantiated the interpretability hypothesis. Nonetheless, the advanced L2ers showed a conservative behavior in the mass context. They significantly opted for a/an in wide scope indefinite non-referential de/re context. The findings reveal that article suppliance creates more learnability problems in the plural and indefinite mass contexts compared to the count singular ones.
