Allan, D. (2004). Oxford placement test. Oxford: Oxford University Press.
Browning, M. (1987). Null operator constructions. Unpublished PhD dissertation, Massachusetts Institute of Technology, Cambridge, MA.
Cardinaletti, A., & Starke, M. (1999). The typology of structural deficiency: a case study of the three classes of pronouns. In van Riemsdijk, H. (Ed.), Clitics in the languages of Europe (pp. 145-234). Berlin: Mouton de Gruyter.
Chomsky, N. (1977). On wh-movement. In Culicover, P., Wasow, T. and Akmajian, A., (Eds.), Formal syntax (pp. 71-132). New York: Academic Press.
Chomsky, N. (1986). Barriers. Cambridge, MA: MIT Press.
Chomsky, N. (1995). The minimalist program. Cambridge, MA: MIT Press.
Contreras, H. (1993). On null operator structures. Natural Language and Linguistic Theory, 11, 1-30.
Eubank, L. (1994). Optionality and the initial state in L2 development. In Hoekstra, T. & Schwarts, B.D., (Eds.), Language acquisition studies in generative grammar (pp. 369-88). Amsterdam: John Benjamins.
Eubank, L. (1996). Negation in early German-English interlanguage: more valueless features in the L2 initial stage. Second Language Research, 12, 73-106.
Geranpayeh, A. (2006). A quick review of the English quick placement test. Retrieved from University of Cambridge ESOL Examinations: http:// uniss.it/documenti/lingue/
Grimshaw, J. (1997). Projection, heads, and optionality. Linguistic Inquiry, 28, 373-422.
Hawkins, R. & Chan, C.Y. (1997). The partial availability of universal grammar in second language acquisition: the ‘failed functional features hypothesis’. Second Language Research, 13, 187-226.
Haegeman, L. (1997) "N-Words, Indefinites and the Neg Criterion", in D. Forget, P. Hirschbühler, F. Martineau and M. L. Rivero (Eds.) Negation and Polarity. Syntax and Semantics (pp.115-137), Benjamins, Amsterdam.
Hymans, N. (1996). The underspecification of functional categories in early grammar. In Clahsen, H., (Ed.), Generative perspectives on language acquisition (pp.91-127). Amsterdam: John Benjamins.
Jabbari, A. A. (2014). Collocational differences in Persian and English and their effect on learners’ production. International Journal of Educational Investigations, 1, 172-190.
Lasnik, H., & Stowell, T. (1991). Weakest crossover. Linguistic Inquiry, 22, 687-720.
Parodi, T., & Tsimpli, I. M. (2005). Real and apparent optionality in second language grammars: Finiteness and pronouns in null operator structures. Second Language Research, 21(3), 250-285.
Prevost, P., & White, L. (2000). Missing surface inflection or impairment in L2 acquisition? Evidence from tense and agreement. Second Language Research, 16,103-33.
Rasekh Mahand, M. (2009). Persian clitics: Doubling and agreement. Linguistics Research Journal, 2(2), 75-85.
Rebarber, T., Rybinski, P., Hauck, M., Scarcella, R., Buteux, A., & Wang, J. (2007). Designing the comprehensive English language learner assessment (CELLA) for the benefit of users. In J. Abedi (Ed.), English language proficiency assessment in the nation: Current status and future practice (pp. 63-79). Davis: University of California.
Roberts, T. (2000). Clitics and agreement.(Unpublished Ph.D. dissertation). MIT. Boston.
Robertson, D. (2000). Variability in the use of the English article system by Chinese learners of English. Second Language Research, 16,135-72.
Smith, V. N., & Tsimpli, I. M. (1995). The mind of savant: Language learning and modularity. London: Blackwell publisher.
Sorace, A. (1999). Initial states, end states and residual optionality in L2 acquisition. Proceedings of the 23rd Boston University conference on language development, 666-74.
Sorace, A. (2000). Syntactic optionality in non-native grammars. Second Language Research, 16, 93-102.
White, L. (1992). Long and short verb movement in second language acquisition. Canadian Journal of Linguistics, 37, 273-86.
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