Chinese grammar: must use 了1 - Youtube - Cchatty

了1, aspect particle 了 is to indicate the completion of an action.

了1 must be used in the following cases:

1, 了1 is NOT ONLY express the past tense, but also the present tense and future tense.
昨天他走了。
你看，他走了！
明天上午八点， 他一定已经走了。

2, the action is completed at a certain time, there is a specific time in the sentence.
昨天晚上我给家人打了电话。

3, indicate the sequence of actions.
我下了课就回家。
Verb1 + 了1 + Object + 就 + ……

4, come after the second verb, in the sentence with verbal constructions in series (连动句)
我去超市买了东西。

5, come after the second verb, in the pivotal sentence (兼语句)
（昨天晚上）我请了朋友吃北京烤鸭。

6, used after the complement, directional complement (趋向补语)
我看完了书。
老师从书包里拿出来了一本书。