Dual-route models of second language (L2) morphology (Clahsen & Felser, 2006; Ullman, 2004) argue that adult L2 learners rely on full-form retrieval, and therefore cannot use combination to produce inflected forms. We tested this prediction with learning of Spanish verb conjugations. Beginning (Experiment 1) and intermediate (Experiment 2) learners (total N = 816) completed 80–90 minutes of web-based training, conjugating regular and subregular verbs in present and preterite tense. Tests of generalization items showed that training led to substantial improvement, equally for metalinguistic and analogical feedback. Comparison with an untrained group showed that gains were maintained 18 weeks after training. In contrast with dual-route model predictions, pre-test accuracy and learning gains were strongly predicted by conjugation pattern, showing that full-form retrieval was insufficient to explain learner performance. Results indicate that adult L2 learners apply compositional analysis, and that conjugation patterns are learned on the basis of their relative cue validity.