Current approaches to bilingualism and language learning ability obscure differences between capacity for learning (ability) and dominance (relative proficiency). Bilingual children with developmental language disorder (DLD) have persistent difficulties with complex syntax. The effects of language learning ability and relative proficiency on syntactic development in bilingual acquisition are not well described. This cross-sectional study examined the continuous effects of language ability and relative proficiency on the production of conditionals, subject/object relatives and passives in a sample of 34 five- to nine-year-old Spanish–English bilingual children, 12 of whom were identified as having DLD. Conditionals were significantly easier than other forms, and there were no differences between subject and object relatives. Higher language ability was associated with greater accuracy. Relative proficiency predicted higher English performance for balanced and English-dominant children. Further examination of language ability and relative proficiency in diverse language learners is warranted.
