The research on language acquisition and retention has primarily focused on monolinguals and bilinguals, with comparatively few studies including trilinguals. To address this gap, the current study compares the acquisition and retention of a novel morphosyntactic rule in Spanish in twelve monolinguals, twelve bilinguals, and twelve trilinguals. The participants underwent training on the preterite verb tense in Spanish, and their ability to correctly identify the rule was tested after two days of training. The study also investigated the effect of verb type on the acquisition of novel morphosyntactic rules in monolinguals and multilinguals. The results revealed that all participants performed better during the delayed retention phase than the other phases. However, bilingual participants demonstrated significantly better accuracy in identifying newly learned tense forms than their monolingual and trilingual counterparts. The study also revealed that regular past tense forms were easier to learn than irregular past tense forms. Overall, the findings from this study suggest that individuals who speak more than two languages may face challenges in acquiring additional languages.