Few studies have detailed the emergence and growth of the oral language and discourse characteristics of school-age students at different grades and across time (Bailey, 2010; Hoff, 2013). Yet general education teachers and English language specialists need well-articulated, preferably empirically derived progressions of language learning to support their students’ oral language development, particularly with those students who are acquiring English as a new or second language. Explanation skills, which encompass the development of many different language and discourse features, were the focus of this study. The language learners whose elicited oral explanations were used to characterize the order of emergence of features were in kindergarten and third grade (5–6 and 8–9 years old). These students were acquiring English in school with predominantly Spanish as their first language. Development of explanations by a cohort of English monolingual or proficient students provides grade-specific comparison language trajectories. The results are placed within the context of a formative assessment framework in order to assist teachers and students in placing explanations at different phases of sophistication on the progressions. This facilitates charting development and identifying with greater specificity which language and discourse features might be effective targets of contingent instruction to promote students’ oral explanation abilities.