This thesis investigates the development of the weak language in early bilingual language acquisition and its results are based on longitudinal and experimental data from 4 Italian-English bilingual children and their parents. The purpose of this thesis is twofold: firstly, to present a new method to assess weak language development and the role of the input in bilingual first language acquisition; secondly, to determine whether there is a relationship between input, weak language development and the acquisition of new information structure. The factors included in the analysis of the weak language are rate of acquisition, production of target-deviant forms, vocabulary, MLU and discourse pragmatics. The results are summarised in the Weak Language Scale. The results are further tested by examining longitudinal and experimental data which are used to test the hypothesis that children who develop Italian as a weak language have difficulty processing subject inversion structures, which require a high processing load due to the interface between syntax and pragmatics. The results of the Weak Language Scale are then compared to those of the Input Scale, which represents the amount of qualitative and quantitative input each child has been exposed to. The final results show that the input plays a major role in bilingual first language acquisition and it has an effect on weak language development. The findings also suggest that linguistic properties at the interface between syntax and pragmatics are harder to process for children who develop Italian as a weak language.