Studies of the second language acquisition of pronominal arguments have observed that: (1) L1 speakers of null subject languages of the Spanish type drop more subjects in their second language (L2) English than first language (L1) speakers of null subject languages of the Korean type and (2) speakers of Korean-type languages drop more objects than subjects in their L2 English. An analysis of these two asymmetries is conducted within the Minimalist Program framework (MP), which hypothesizes that language acquisition involves the learning of formal features of a target language. I propose, based on Alexiadou and Anagnostopoulou (1998), that the licensing of null subjects is conditioned by the interpretability of agreement features. When a language has [+interpretable] agreement features, raising of the verb to T (X-movement) satisfies the EPP requirement: hence, a null subject is allowed. On the other hand, in a language with [-interpretable] agreement features, the subject is obligatory since merger of the subject in the specifier of TP (XPmerge) is required to check the EPP feature. Learning of the obligatory status of English subjects is easier for Korean learners than for Spanish speakers since syntactically both English and Korean have the same feature value [-interpretable] (although null subjects are allowed in Korean for pragmatic reasons). Spanish has the opposite syntactic feature value [+interpretable] and resetting of this is more difficult. Licensing of null objects is hypothesized to be related to the strength of theta-features. Languages with strong theta-features, such as English and Spanish, do not allow null objects, whereas languages with weak theta-features like Korean allow null objects. It takes time for Korean speakers to learn the different value of English theta-features, resulting in the extended null object period in L2 English of Korean L1 speakers.