Children start producing ["necessary and sufficient"] conditionals relatively late. Past counterfactuals (PCFs), for example "If she had shut the cage, the rabbit wouldn't have escaped", are particularly problematic for children; despite evidence of comprehension in the preschool years, children aged eleven are still making production errors in PCF structure (Crutchley, 2004). Working within a usage-based framework, the present study explores whether PCFs in the conversational component of the British National Corpus show structural similarities to the set of PCF structures produced by six- to eleven-year-old children in an elicitation task. Adult PCFs are found to be both rare in spontaneous conversation and very varied in structure. Low token frequency and high type frequency are hypothesized to account partly for children's late acquisition of the PCF construction. However, regularities in the use of subjects and verbs in adult PCFs are hypothesized to assist children's acquisition of the construction.