了(le) is one of the most frequently used function words in modern Chinese language, by itself, it has no concrete meaning, rather, it’s a ‘helper’ word used in many grammatical functions. Here are some important ways that 了 is used.
Completion of an Action
了 can be used after a verb to indicate the completion of an action
Because le is used most commonly for this purpose, elementary Chinese learners come to understand it simply as equivalent to English past tense. According to this understanding, verb +了 equals past tense, but that’s not accurate, because a verb’s completion can take place in the past, in the present, or in the future.
Last summer, they had a child. past
Now, they have a child. present
After having a child, we won’t have as much spare time. future
Some Chinese learners assume that, since they just need put 了after the verb to render Chinese sentence in the past tense, then this rule must suit all Chinese verbs. Unfortunately, it’s not that simple; there are some rules, which must be observed:
1. Some verbs that describe a mental state or emotional feeling can not be followed by 了 to express past tense.
She felt very sad yesterday.
Last year, she hoped to come back to Shanghai.
Other verbs in this category: 想念 (miss)，喜欢 (like)，爱 (love)，盼望 (look forward)
2. In a past tense sentence, if the object is an subordinate clause, the verb can not be followed by 了.
Many years ago, I realized that I loved Shanghai.
Yesterday, I swore I would study Chinese very hard.
3. In a dual-function constituent sentence, the first verb cannot be followed by 了.
He asked me to forgive him.
Indication of a Change
了 can be used at the end, or in the pause position of a sentence to indicate a change, or that new information or a new situation has appeared - as the word ‘now’ is often used in English.
He got fat.
This means he was not fat before.
It is spring now, and the flowers are in bloom.
This means the flowers were not in bloom before.
He is a teacher now.
This means he was not teacher before.
My son can sing now.
Thich means my son could not sing before.
Express Urging, Dissuasion, or Reminder
了 can be used at the end, or in the pause position of a sentence to express urging, dissuasion or reminder.
Go! Go! It’s already 8 o’clock!
2. 够了！不要再提这件事了！ (dissuasion)
Enough! Don’t mention this ever again!
Work is finished! Let’s go home.
Fixed Phrases using 了
1. 太……了。Means ‘very’ or ‘very much’.
今天太热了。It is very hot today.
她太漂亮了。She is very beautiful.
我太喜欢上海了。I like Shanghai very much.
2. 该……了。Means: “It’s time to (action verb).”
该上班了。It’s time to go to work.
该睡觉了。It’s time to go to sleep.
该吃晚饭了。It’s time to have dinner.
3. 要……了 Means someone or something “is going to do (something).”
爸爸要去上班了。Dad is going to go to work.
天要下雨了。 It is going to rain.
4. 快(要)……了 Means something “will happen soon.”
我快30岁了。 I will soon be 30 years old.
圣诞节快来了。 Christmas is coming soon.
我快要冻死了。 I will soon freeze to death!