Adult learners have persistent difficulty processing second language (L2) inflectional morphology. We investigate associative learning explanations that involve the blocking of later experienced cues by earlier learned ones in the first language (L1; i.e., transfer) and the L2 (i.e., proficiency). Sagarra (2008) and Ellis and Sagarra (2010b) found that, unlike Spanish monolinguals, intermediate English-Spanish learners rely more on salient adverbs than on less salient verb inflections, but it is not clear whether this preference is a result of a default or a L1-based strategy. To address this question, 120 English (poor morphology) and Romanian (rich morphology) learners of Spanish (rich morphology) and 98 English, Romanian, and Spanish monolinguals read sentences in L2 Spanish (or their L1 in the case of the monolinguals) containing adverb-verb and verb-adverb congruencies or incongruencies and chose one of four pictures after each sentence (i.e., two that competed for meaning and two for form). Eye-tracking data revealed significant effects for (a) sensitivity (all participants were sensitive to tense incongruencies), (b) cue location in the sentence (participants spent more time at their preferred cue, regardless of its position), (c) L1 experience (morphologically rich L1 learners and monolinguals looked longer at verbs than morphologically poor L1 learners and monolinguals), and (d) L2 experience (low-proficiency learners read more slowly and regressed longer than high-proficiency learners). We conclude that intermediate and advanced learners are sensitive to tense incongruencies and--like native speakers--tend to rely more heavily on verbs if their L1 is morphologically rich. These findings reinforce theories that support transfer effects such as the unified competition model and the associative learning model but do not contradict Clahsen and Felser's (2006a) shallow structure hypothesis because the target structure was morphological agreement rather than syntactic agreement.