“…A MAJOR FOCUS OF SECOND LANGUAGE acquisition (SLA) research to date has sought to understand the competition and relationships between a learner's different languages (Calabria et al., ). This research has repeatedly shown that the use of a single language activates a speaker's other known languages (Marian & Spivey, ; Wu & Thierry, ), that prior first language (L1) knowledge and experience can influence second language (L2) use (e.g., selective attention to linguistic cues; Ellis & Sagarra, ; MacWhinney, ), and that L1–L2 differences can influence the route and rate of L2 morphosyntactic development and processing (Avery & Marsden, 2019; Isabelli, ; McManus, , ; Murakami, ; Roberts & Liszka, ). However, despite major advances in what we know about the cognitive effects and mechanisms of learning a second language, little research has systematically examined the next step in this program: How can this understanding about the competition and relationships among a learner's different languages be used to facilitate L2 learning and teaching?…”