When a le 了 is put behind the verb, it can be used as an aspect marker to show the completion of an event at a perfect aspect, which can be used to talk about something that happens in the past, present or future, so it does not necessarily mean the event happened in the past like the tense marker for past tense in English.
E.g.: Past:「我吃了湯圓」’I ate the sweet dumplings.’
Future:「我下了課後就會打電話給你」’I will call you after my lessons end’
Le 了 can also be put at the end of the sentence as a modal particle to show the current relevance state, which has five categories, including change of state, contradict assumption, implication of new situation, statement closure or the report of the progress so far ( Li, & Thompson, 1981).
E.g.:「他病了」‘He is sick.’
The le了in this sentence can show the change of state that he is sick now but was not before.
Le 了can also be used as a sentential particle to strengthen the tone of the sentence, meaning extremely or very.
E.g.:「太美味了」. ‘It is very delicious.’
The le了in this sentence can strengthen and add degree to the adjective 「美味」accompanied by the word tai太.
Sometimes, we may put two le了in one sentence. This implies that the event is complete and is what has happened up until now or so far. The action in the sentence with double le了may continue as it depends on the follow-up of the sentence. (Chinese e-Learning Center, NOU (TW), 2021)
E.g.:「他吃了兩碗飯了」‘He has eaten two bowls of rice already.’
The action of him eating two bowls of rice has already been done and it is the situation so far.
Chinese e-Learning Center, NOU (TW) 國立空中大學數位華語文中心. (2021, December 29). Mandarin Grammar: Ep25. Single 了(le) and double 了(le) [Video]. YouTube. https://www.youtube.com/watch?v=j0QkMxsqR5s
Li, & Thompson, S. A. (1981). Mandarin Chinese : a functional reference grammar. University of California Press.