了le is very difficult to grasp in Chinese. It can appear after a verb or at the end of a sentence, and serves a variety of functions.

# Post-Verbal (Perfective) 了
The verbal 了 is used primarily to mark the completion [[Aspect|aspect]] of a verb. Note that this is not equivalent to marking a past action (tense).
In English, the "perfective"/"completive" may appear in all three tenses:
1. Past Perfect: I *had* already arrived (when he called).
2.