This paper examined lag effects in the learning of second language (L2) grammar. Moreover, following the Desirable Difficulty Framework for L2 practice, the present study investigated whether lag effects could be explained by other sources of difficulty. Using digital flashcards, 117 English language learners (aged 10–18) learned two grammatical structures over two different sessions at a 1-day or 7-day intersession interval (ISI). Learners’ performance was analyzed at two retention intervals (RIs) of 7 and 28 days, respectively. Linguistic difficulty was compared by examining two different structures, while learner-related difficulty was analyzed by comparing learners who differed in terms of age, proficiency, and time required to complete the training. Results showed no main effect of ISI, a main effect of RI, and a small but significant ISI × RI interaction. Linguistic difficulty and age did not interact with ISI or RI. However, longer lags led to significantly higher scores for faster learners and learners of higher proficiency, while shorter lags promoted significantly higher scores for slower learners and learners of lower proficiency. The findings provide some support for the Desirable Difficulty Framework in its potential to explain L2 lag effects.

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This study investigated how much practice is necessary for learners to attain durable second language (L2) grammar knowledge. Using digital flashcards, 119 participants practiced translating 12 sentences into an artificial language, followed by feedback, until they had typed all sentences correctly. Participants repeated this activity in one, two, three, or four relearning sessions on consecutive days. After a 14‐day delay, all groups scored highly on a receptive test. However, scores on a productive test were substantially higher for groups with three or four relearning sessions. Accuracy tended to peak on the 3rd day of training. An analysis by individual training performance revealed that participants attained durable productive knowledge if they completed two sessions without errors, regardless of how many sessions they had performed in total. The findings provide a timeframe for processes described in skill retention theory (Kim et al., 2013) and suggest a performance benchmark to indicate when learners have gained procedural L2 grammar knowledge.

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