Monolingual English 4-year-olds were administered a Subject-Verb agreement comprehension task that included stimuli such as the boys spinø versus the boyø spins (…/freely/in the hall). They were categorized as users of Mainstream American English (MAE)(N= 8), Some Variation (N= 9) and Strong Variation (N= 9) based on the Diagnostic Evaluation of Language Variation (Seymour et al., 2005). Only MAE users performed significantly above chance across conditions (final versus medial with adverb versus medial with prepositional phrase) while comprehension in learners of other varieties was sensitive to syntactic context. Corpus analyses revealed that input frequency cannot explain our comprehension results and confirmed the plausibility of the hypothesis that 3rd person-s competes with other structures to express generic tense in non-mainstream varieties of English.