Previous research has indicated that explicit information (EI) about either the first language (L1) or second language (L2) along with task-essential practice can facilitate L2 learning (e.g., Fernández, 2008; McManus, 2022). However, little research has examined L1--L2 contrastive EI with L1/L2 practice. Targeting plural-marking accuracy, the present study sought to fill this gap by exposing 127 Japanese intermediate learners of English to six online treatment sessions across four conditions: (1) non-contrastive EI (pluralization rule explanation) + L2 practice, (2) contrastive EI + L2 practice, (3) as per Condition 2 + additional L1 practice (application of L2 pluralization to L1 through the Japanese suffix --"tachi"), and (4) using prepositions as a control, non-contrastive EI (prepositional rule explanation) + L2 practice. Accuracy was tested at pre/post/delayed intervals using an acceptability judgement task (reading) and a picture description task (writing). Statistical analyses demonstrated greater gains from the contrastive variables in the immediate post-test, with L1 practice appearing especially beneficial, despite improvements diminishing in the delayed post-test. Supplementary data from retroactive interviews and questionnaire responses indicated favourable orientations towards the intervention. The study advances understanding of how EI and practice type can be used to maximize L2 learning through web-based practices tailored to the nature of the learning problem.