This paper explores whether the poor mastery of morphosyntax exhibited by second language (L2) learners can be tied to difficulties with non-syntactic processing. Specifically, we examine whether problems with English regular and irregular past tense are related to poor L2 phonological ability and lexical access, respectively. In Experiment 1, L2 learners showed poorer past tense mastery than native English speakers in grammaticality judgment and production tasks. L2 phonological ability was positively correlated with correct performance on regular verbs and negatively with unmarked production. L2 lexical access was positively correlated with correct performance on irregular verbs, and negatively with overregularization production. Experiment 2 simulated these difficulties in native English speakers by placing them under phonological processing (noise) or lexical access (deadline) stress. Noise selectively impacted regular verbs in grammaticality judgment but impacted all verb types in production. Deadline pressure impacted irregular verbs while sparing regular verbs across both tasks. Thus, non-syntactic processing difficulties can have specific impacts on morphosyntactic performance in both non-native and native English speakers.