Abstract:This paper holds that the key reason for the complexity of the study of lemay lie in its intricate relationship with the syntactic and discourse environment, which leads to the difficulty of distinguishing its proper meaning from the interactive meaning with the context, and that the "past" expressed by lecan be construed into three different senses:"the inferred past", "the conceived past" and"the synchronized past".Hence, leis a multifunctional grammatical marker integrating tense, aspect and modality.This paper further argues that leexpresses the internal time of events (aspect) through interaction with the syntactic and discourse environment, and the external time relationship (tense) of the event through interaction with temporal reference points into different "pasts", which can effectively account for those various "exceptional uses".The modality of lecomes from temporality itself as a modality species.Besides, the tense and aspect marking model in Mandarin Chinese adopted in this paper can effectively explain such puzzles as sentence completion and categorization of le, etc.The unified account of the multifunctionality of tense-aspect-modality of lecontributes to the economy of theoretical explanation and has important implications for language teaching.