When processing sentences about perceptible scenes and performable actions, language understanders activate perceptual and motor systems to perform mental simulations of those events. But little is known about exactly what linguistic elements activate modality-specific systems during language processing. While it is known that content words, like nouns and verbs, influence the content of a mental simulation, the role of grammar is less well understood. We investigate the role of grammatical markers in mental simulation through two experiments in which we manipulate the meanings of sentences by modifying the grammatical aspect they use. Using the Action-sentence Compatibility Effect (ACE) methodology [Glenberg, A., Kaschak, M. (2002). Grounding language in action. "Psychonomic Bulletin and Review, 9", 558-565], we show that progressive sentences about hand motion facilitate manual action in the same direction, while perfect sentences that are identical in every way except their aspect do not. The broader implication of this finding for language processing is that while content words tell understanders what to mentally simulate and what brain regions to use in performing these simulations, grammatical constructions such as aspect modulate how those simulations are performed. (Contains 2 tables and 4 figures.)