Past research has emphasized the universality of grammar acquisition over key differences, resulting in the development of a number of one-size-fits-all approaches to grammar instruction. Because such approaches fail to consider disparities of grammatical features, they are often ineffective. Just as a doctor needs to diagnose an illness and prescribe a suitable treatment, the teacher must evaluate a grammatical feature and choose an appropriate instruction. To better understand how this may be accomplished, highly disparate grammatical features (the definite article and plural noun) were taught to adult second language learners, using three different pedagogical techniques: Explicit focus on meaning, explicit focus on form, and implicit focus on form. Results suggest that the effectiveness of these treatments depends upon characteristics of the grammatical feature, the type of instruction utilized (implicit or explicit), and the learner's language proficiency. According to the results, an empirical method to guide the content of grammar instruction is proposed.