了 is one of the most powerful words in Chinese. 了 is powerful because it can act as a verb, adverb, adjective and also auxiliary word. In this post, we will study the use of 了 as an auxiliary word of tense. Remember these two formulas of 了 for its use as an auxiliary word and you won’t feel puzzled any more:
1. Verb/adjective + 了 = is/get/become/go/turn
This formula of the use of 了 expresses completion of a change of state, property or action. For example:
I’ m tired because I’ve just run home from school. 我累了，因为我刚从学校跑回家。
So many sorrows turned her hair white. 这许许多多不幸的事使她的头发变白了。
Let s shelter under this tree until the rain stops. 咱们在这棵树底下躲到雨停了再走。
As one gets old, one does not remember as one used to do. 人老了，记忆力就不如从前了。
2. Verb+ 了 = did/have done sth.
This use of 了 is exactly the formula of the simple past tense or perfect tense in Chinese. Just add 了 to a sentence to express that something happened/has happened, for example,
他昨天晚上请我去吃饭了。He invited me to dinner last night
我今天早晨给他打电话了。I called him this morning.
然而，当她大声嘲笑起来的时候，她们的脸不禁涨红了。They flushed, however, when she burst into a shout of derisive laughter.
玛丽工作结束以后，回家去了。After Mary finished her work, she went home.
他不可能做完了家庭作业。He can’t possibly have finished his homework.
她过去十分腼腆，但在国外呆了一年以后完全变了。She used to be terribly shy, but a year abroad have completely transformed her.
The above uses of 了 are very important as you see that the two formulas can apply universally to adjectives and verbs to express a change of state, property or action, and also form the simple past or perfect tense. So, you can say 了 is a symbol of the perfect tense.