This study investigates the effects of focus on form on the acquisition of the tense-aspect system in English by comparing two different instructional techniques, implicit and explicit focus on form. Furthermore, it examines which class of lexical aspect use is more applicable to each method of instruction. The implicit method is intended to produce the synergy of input flooding and typographical enhancement technique. The explicit method provides learners with pedagogical statements, corrective feedback, and some exemplars which are neither flooded nor visually enhanced. The subjects were Korean junior college students enrolled in two basic English writing classes. One class was assigned to an implicit group and the other to an explicit group. They received instruction for 10 weeks under two different learning conditions and were tested on the target structures. The results showed the overall effectiveness of implicit and explicit focus-on-form instructions on facilitating the two target structures, the simple past and present perfect, and on the lexical aspectual system as well. Although the implicit group made some progress, the gain was not statistically significant. The explicit group, however, made a significant progress in the learning of the present perfect. As for the lexical aspectual system, the implicit instruction somewhat influenced appropriate use of telic verbs, whereas the explicit instruction was more effective in facilitating the appropriate use of atelic verbs. The results indicate the importance of acquisition order and learners’ readiness in the learning of the present perfect.