The most important distinction is the perfective and the imperfective aspect. The perfective aspect views a situation as ‘complete, total and whole’, or put in other words, the whole of the situation is presented as a ‘single unanalyzable whole, with beginning, middle, and end rolled into one, and no attempt is made to divide this situation up into various individual phases’ (Comrie, 1976). The imperfective is an aspectual viewpoint that refers to the internal temporal structure of a situation, viewing the situation from within, and presenting it as ongoing or continuous without inclusion of its beginning or end. Therefore, the aspectual distinction between the perfective and the imperfective is that the former views a situation as complete and closed (namely, with both initial and final points of the situation); whereas the latter views a situation as incomplete and open (namely, with neither the initial nor the end point of the situation). To illustrate the distinction between the perfective and imperfective aspect, please consider:
Wǒ huí jiā de shíhou, nǚér zhèngzài zuò zuòyè.
(When I got home, my daughter was doing her homework.)
Wǒ huí jiā de shíhou, nǚér yǐ zuò le zuò yè.
(When I got home, my daughter already did her homework.)
Both in (1a, b), the event of ‘doing homework’ is in the past tense, but (1a) differs from (1b) in terms of the perspective from which the event is viewed. In (1a), the main clause is in the progressive aspect, expressed by the marker ‘(zhèng) zài’. The progressive aspect does not present the situation of ‘doing homework’ in its entirety, but instead makes explicit reference to the internal makeup of ‘doing’, presenting it as incomplete and ongoing. The aspectual viewpoint does not include the initial or end points of the event, that is to say, the clause tells us nothing about when the event was started or when it was completed. In contrast, the main clause in (1b) is in the perfective aspect ‘le’, which presents the event of ‘doing homework’ as complete (in Chinese, the perfective signals ‘completion’, too), thus as a single whole, including the initial and end points of the event.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.