arXiv:2610.41262v1 Announce Type: cross Abstract: Data-driven learning for the solution operator of a differential equation has broad scientific importance, but observations from a target system are often scarce and costly to collect. Related source systems may provide abundant observations under similar physical conditions, whereas even small changes in the governing equation can induce complex changes in the solution operator, making naive data pooling ineffective. We propose a structured transfer learning method that accounts for unknown sparse differences in the governing equations from a known library of candidate terms. Using the resolvent identity and a whitened instrumental variable approach, we construct unbiased source estimating equations for the target operator. Building on these equations, we develop a three-stage estimator that consists of a target-only ridge pilot, source-wise sparse calibration, and a projected refit designed to remove the leading calibration error inherited from the pilot. An iterative recalibration scheme further reduces the remaining error. We establish non-asymptotic prediction bounds showing when transfer achieves the pooled all-source rate, together with minimax lower bounds that characterize both rate optimality and fundamental identifiability limits under common shifts. Simulations and an electronic band-pass filter application demonstrate clear gains over target-only learning, naive pooling and existing transfer methods.