D. By being the first event in a sequence. ('Verb + le 了 + unquantified/unmodified object' means 'after [doing] ...'. It must be followed by another action.)

  1. Míngtiān wǒ chī-le wǎnfàn jiù qù xuéxiào.
    明天我吃了晚饭就去学校。
    Tomorrow I will go to school after I eat dinner.

    If Sentence 1 ended at wǎnfàn 晚饭, that is Míngtiān wǒ chī-le wǎnfàn 明天我吃了晚饭, it would mean'tomorrow after I eat dinner', which is not a complete sentence.


  2. Zuótiān wǒ chī-le wǎnfàn jiù qù xuéxiào le.
    昨天我吃了晚饭就去学校了。
    Yesterday I went to school after I ate dinner.

    The sentence-end le 了 in 2 is a modal particle signifying the completion of the second action. The only difference between 1 and 2 is the use of the modal particle le 了 at the end of 2. In 1 neither action has taken place yet, while in 2 both actions have. However, the aspect particle le 了 in both sentences signifies that the verb it is attached to is the first action in the sequence.