An important issue in second language (L2) acquisition is understanding the conditions under which successful development occurs. Of particular interest is whether explicit or implicit conditions are more beneficial for development and whether this depends on the linguistic structure (e.g., complex forms) or individual differences (e.g., working memory). The present study will explore the relationship between training condition and individual differences for the development of a complex L2 form by examining the Spanish present subjunctive in expressions of doubt under three training conditions--explicit (comprised of explicit information and structured input practice), implicit (comprised of meaningful exposure and structured input practice), and a control condition (comprised of meaningful exposure). Measures of working memory (i.e., complex span tasks) were administered to examine the role of individual differences. Results indicated an advantage for the explicit condition at the immediate but not delayed post-test. In contrast, the implicit condition showed smaller gains but maintained them over time. Regarding individual differences, working memory positively related to development under the explicit condition, but was negatively related to development under the implicit condition. Thus, we provide evidence that learners' individual differences and the type of training they are exposed to may influence development of a complex form.