Background: Individuals with Down syndrome (DS) experience difficulties with receptive and expressive grammar and specifically morphosyntax. Despite these difficulties, there have been few studies to evaluate the effectiveness of intervention and limited evidence of generalisation to untaught items. /

Aim: To evaluate the efficacy of a language intervention on the acquisition of the regular simple past tense (RSPT) in children with DS aged 7-11 years and to explore whether any gains in the use of this grammatical rule will generalise. /

Method: A randomised controlled trial evaluated a 10-week intervention, using explicit and implicit methods, designed for children with DS. Fifty-two children with DS aged 7-11 years were randomly allocated into two groups: 1) intervention group and 2) delayed intervention group. All children were assessed at three timepoints: preintervention (t1), after the intervention group had received the intervention (t2), and 12-14 weeks later (after the delayed intervention group had received the intervention) (t3). The intervention was delivered by trained teaching assistants (TAs) in daily 20-minute sessions. /

Results: The intervention group made significantly greater gains at t2 on a composite measure of the use of the RSPT (d=1.63). These gains were maintained 12-14 weeks later at t3 when the delayed intervention group also made similar gains. The use of the RSPT generalised to untaught regular verbs. In addition, the children made errors of overregularisation on irregular verbs demonstrating they had learnt the grammatical rule. Generalisation to other tense morphemes (e.g., the third person singular) did not occur. /

Conclusions: An intervention, using explicit and implicit methods, was successful in teaching children with DS to use a grammatical rule. Furthermore, the children were able to generalise this rule to untaught items. This provides evidence for intervention targeting morphosyntax and the feasibility of training TAs to deliver this intervention.