Results: Parents' use of copula is with contraction to pronominal subjects outnumbered uses with lexical subjects or without contraction. Parents' use of the less common copulas, with low-frequency lexical subjects or without contraction to the subject, was significantly positively related to children's subsequent growth in unique subject+copula combinations. However, parents' use of copulas contracted to pronouns was not related to children's copula productivity. Conclusions: These findings hold implications for understanding how children make use of grammatical information in parent input, for how to measure children's grammatical growth, and for enhancing the copula input provided by clinicians and parents.