This thesis aims to expand the testing grounds of the Interface Hypothesis (IH) by investigating the acquisition of English subjects operating with pure syntax and article uses of (in)definiteness and genericity in English governed with external and internal interfaces respectively by L1-Turkish learners. Fifty-eight sophomore students who have been studying in the Department of Foreign Language Education of a state university in central Turkey were asked to complete a Grammaticality Judgement, a Discourse Completion and a Forced-Choice Elicitation Task subsequently. By doing so, the aim was to understand whether proficient Turkish learners of English interpret and use subjects and articles accurately in English and whether the IH can account for the results. To this end, upon analyzing the data, the relationship between the participants’ performances were discussed in relation to the IH. The findings highlighted that narrow syntactic properties are acquired with the least difficulty. The properties at an interface domain, on the other hand, present more difficulty regardless of the type of the interface. Additionally, the IH was found partially accountable for the participants’ performances. It could explain why the participants have done significantly better in subject realization than the articles, whereas it could not account for the lack of statistically significant difference between the (in)definite and generic uses of articles. To build upon the present study, adding tasks measuring language processing might be of great value to gain a better understanding into the reason why more errors are committed when interface structures are involved.