This study presents an analysis of patterns of morphological case in Finnish within the Principles and Parameters framework. Finnish has a rich system of inflection for both case and agreement, making it an important language for testing hypotheses about the relationships between morphological case and abstract Case, and Case/case and agreement. The focus of the study is a set of syntactic environments where internal DP arguments appear in nominative case, but alternate with accusative pronouns. In the same contexts, internal arguments may also receive partitive case to encode features related to aspect or indefiniteness. Because these environments lack an external argument coindexed with agreement, the data is particularly relevant to predictions made by Burzio's Generalization. By testing Burzio's hypothesis systematically against a range of sentence types, Finnish is shown to contain an ergative case subsystem within a nominative-accusative main system. The assignment of the objective cases is linked with the licensing of aspectual roles at D-structure, and finite Tense is posited as a bi-unique Case assigner. The case split then arises as the result of two case features being assigned simultaneously to an internal argument, objective Case at D-structure associated with aspect, and nominative Case at S-structure associated with finite Tense where an external argument is not available. Morphological spell-out rules for particular argument types are proposed which determine the surface case realization of doubly-case assigned nominals.