了
RuleVerb + le: marks completion or realization of an action (aspect, not tense).
Curiosity
Chinese verbs do not change for past tense. So how do you tell "ate" from "eat"?
Intuition
le is not about time but about completion: the action happened and finished. Placed right after the verb, it stamps "this was realized".
Visualization
wo chi fan (I eat) → wo chi le fan (I ate). le attaches after the verb chi to mark completion.
Essence
le is the perfective aspect. It can even apply to future events meaning "once completed" (xia le ke, once class ends). The negative uses mei(you) and drops le (mei chi).
Examples
我吃了饭。
I ate.
他买了一本书。
He bought a book.
我没吃饭。
I did not eat.
Mini-quiz
Which expresses "I watched a movie"?