We analysed both structural and functional aspects of sentences containing the four adverbials "after", "before", "because", and "if" in two dense corpora of parent-child interactions from two British English-acquiring children (2;00-4;07). In comparing mothers' and children's usage we separate out the effects of frequency, cognitive complexity and pragmatics in explaining the course of acquisition of adverbial sentences. We also compare these usage patterns to stimuli used in a range of experimental studies and show how differences may account for some of the difficulties that children have shown in experiments. In addition, we report descriptive data on various aspects of adverbial sentences that have not yet been studied as a resource for future investigations.