This dissertation explores why grammatical structures are better learned implicitly or explicitly by investigating differential object marking (DOM) in Spanish. To address the effectiveness of implicit versus explicit instruction in the classroom, the researcher focused on two of the main features that constrain DOM in Spanish: animacy and specificity. It investigates the relationship between these two features and the two different instructional methodologies. Forty-two (n = 42) second language (L2) English-speaker learners of Spanish received implicit or explicit instruction in the classroom. A third group did not receive any instruction, and a group of native speakers served as a control baseline. The participants completed one pretest and two posttests (one immediate and one delayed). The tests included three tasks that investigated the participants' comprehension, intuition, and production of DOM in Spanish before and after instruction. The statistical results favored explicit instruction over implicit instruction. Additionally, the research further explains why DOM is better learned implicitly or explicitly. The findings show positive results for the explicit group's animacy acquisition. In contrast, specificity poses difficulties for L2 learners. This dissertation contributes to L2 instructional research by providing more information on how the semantic features of a grammatical structure directly affect the effectiveness of explicit and implicit instruction. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]