Obligatory grammatical categories and the expression of temporal events.

Thai has imperfective aspectual morphemes that are not obligatory in usage, whereas English has obligatory grammaticized imperfective aspectual marking on the verb. Furthermore, Thai has verb final deictic-path verbs that form a closed class set. The current study investigated if obligatoriness of these grammatical categories in Thai and English affects the expression of co-occurring temporal events and actions depicted in three different short animations. Ten children aged four years, five years, six years and seven years, and ten adults as a comparison group from each of the two languages participated. English speakers explicitly expressed the ongoingness of the events more than Thai speakers, whereas Thai speakers expressed the entrance and exit of protagonists depicted in the animations significantly more than English speakers. These results support the notion that obligatory grammatical categories shape how Thai and English speakers express temporal events or actions.