Complex sentences involving adverbial clauses appear in children’s speech at about three years of age yet children have difficulty comprehending these sentences well into the school years. To date, the reasons for these difficulties are unclear, largely because previous studies have tended to focus on only sub-types of adverbial clauses, or have tested only limited theoretical models. In this paper, we provide the most comprehensive experimental study to date. We tested four-year-olds, five-year-olds and adults on four different adverbial clauses (before, after, because, if) to evaluate four different theoretical models (semantic, syntactic, frequency-based and capacity-constrained). 71 children and 10 adults (as controls) completed a forced-choice, picture-selection comprehension test, providing accuracy and response time data. Children also completed a battery of tests to assess their linguistic and general cognitive abilities. We found that children’s comprehension was strongly influenced by semantic factors – the iconicity of the event-to-language mappings – and that their response times were influenced by the type of relation expressed by the connective (temporal vs. causal). Neither input frequency (frequency-based account), nor clause order (syntax account) or working memory (capacity-constrained account) provided a good fit to the data. Our findings thus contribute to the development of more sophisticated models of sentence processing. We conclude that such models must also take into account how children’s emerging linguistic understanding interacts with developments in other cognitive domains such as their ability to construct mental models and reason flexibly about them.