The present study examines the outcome of teaching second language English (L2 English) third-person singular present tense -s for declarative knowledge in two different settings, namely Sweden and Vietnam. Third person -s is one of the most studied grammatical structures in second language acquisition research. Data are collected in two classroom contexts with different teaching traditions. Participants are 76 young learners, 11–12 years old, 32 with Swedish as L1 and 44 with L1 Vietnamese. The results demonstrate differences in learner outcomes in these two groups. The Vietnamese learners outperformed the Swedish learners on the tests of declarative knowledge. The analyses of learner explanations of third-person singular present tense -s reveal that learners tend to decompose this complex rule into several features, which sometimes lead them into misconceptions. This has pedagogical implications about EFL teaching and learning for young learners.