This study investigates differential effects of form-focused instruction (FFI) to a simple versus complex L2 targets when the FFI is offered within a context of primarily meaning-focused instruction (MFI). Forty-seven Korean-speaking adult learners of English participated in the study and were randomly assigned to one of the two experimental groups or one control group. The experimental participants were instructed in one simple and one complex targets through MFI plus FFI or MFI only. No instruction was provided for the controls. Learning was measured by a grammaticality judgment task, and knowledge types were examined by subjective measures of awareness. The results indicate that the combination of FFI and MFI results in more robust learning effects than the exclusive use of MFI particularly for both simple and complex targets. As for the types of knowledge, the analyses found that FFI positively influences development of implicit and explicit knowledge.