“了” is an inflectional morpheme, a perfective aspect marker attached to free roots. It is not a tense marker since there is no tense in Chinese. Unlike English, whether the event is in the past, present, or future, it is contextually determined that “了” does not affect the word class of its stem. It is more similar to an indicator of an event. For example, in the sentence “我明天到了再跟你說。”, the event ‘到了’ has not happened actually but happened in the future world because of the trigger “明天”, but “到了” here is not a tense marker since it is not semantically identical with the meaning of “will arrive”. Without the trigger to mention the specific time, the expressions with “了” in Chinese can only be interpreted as past or present perfect tense in English. For example, “我寫了一封信。” can be interpreted as “I wrote a letter.” or “I have written a letter.”, which happened in the past.
The use of “了” is involved in four types of events. The first type is “quantified events”, it is a bounded event that contains a quantifier like the number of times the event happened, the elapsed time, or the extent to which the event occurs. For examples, “我喝了兩杯水”, “他坐了五分鐘” and “他長高了很多”. As for “definite or specific events”, they are events that the object is a definite noun phrase. For instance, “我看見了小明”, which “小明” refer to a specific person. The “first event in a series” happens when the events are bounded by the following events in the same sequence. For example, “我吃了飯就上學”, “了” here is inserted in the first event rather than the second event, which “吃飯” is bounded by “上學”. Finally, “events with a change of situation" usually involve a stative verb, and the event causes it to change its state from A to B. For instance, “你醉了。” indicates the state of “你” change from “not drunk” to “drunk”.
Moreover, There are three types of sentence structures in which 了 can be used, which are [X-le], [[X-le]-NP], and [[[X-le]-NP]x -le] respectively. The examples are “做了”, “做了功課”, and “做了功課了” correspondingly. X represents an event or a state, typically a VP. “了” put after an X indicates that X is realized.
Chao, Y. R. (1968). A grammar of spoken Chinese. University of California Press.
Li, C. N., & Thompson, S. A. (1981). Mandarin Chinese : a functional reference grammar. University of California Press.