Chinese grammar: 了1 (Aspect Particle 了) - Youtube - Cchatty

了1 (Aspect Particle, 动态助词）

了1 is an aspect particle used to express the completion of an action.

1)了 indicates completion in the past. It can also indicate the expected or supposed completion in the future. The pattern ……Verb + 了…… 就…… is often used.

2) In the pattern "Verb1 + 了……（就）Verb2" if the verb1 goes with an object, the object can be a simple form. Here the object is not the key point of the meaning of the sentence. 

3)The difference between "Verb + 了+ measure word + object " and " Verb (了) + measure word + (object) + 了" is that the first indicates that the event has happened and completed while the latter indicates that part of the event has completed or the event has been happening for a period of time and it is likely to continue. 

the negative form of "Verb + 了1" --> "没 + Verb" 吃了饭 --> 没吃饭 

the difference of 了1 (Aspect Particle, 动态助词) and 了2 (Modal particle, 语气助词): https://www.cchatty.com/grammar/10375