The present study explores subject expression among native speakers (NS) and non-native speakers (NNS) of Spanish by examining whether the potential form ambiguity and switch reference variables explain the effects of tense, mood, and aspect (TMA) of the verb on the forms of subject expression used (e.g., lexical noun phrase, null subject, personal pronoun). The data come from sociolinguistic interviews completed by graduate-level NNSs and NSs of Spanish. Although NNSs and NSs in the present investigation are significantly different in their use of subject expression forms and some qualitative differences are observed between groups with respect to TMA and potential form ambiguity, the effects found for TMA are largely similar. For these NNSs and NSs, while potential form ambiguity does not explain the effects of TMA for either group, the switch reference variable does. In addition to these findings, the present investigation also contributes to the existing literature on subject expression on Spanish by operationalizing the concepts of ambiguity and redundancy in ways that can be easily replicated in future research, and by further demonstrating that the examination of the full range of forms that occur in syntactically-defined contexts (i.e., the subject position of all tensed verbs), rather than a subset of those contexts (e.g., animate subject pronouns), is profitable.