1.Completion or realization of an action or event.
The particle‘了’occur after the verb or the sentence, or both.
我买了三本汉语书。wǒ mǎi le sān běn hànyǔshū.
I brought three Chinese books.
他去(了)上海了。tā qù shànghǎi le.
He went to Shanghai. (He has been to Shanghai.)
‘了’is not only used in a past tense sentence. Its basic function is to indicate the completion or realization of an action or event.
到了以后，我给你打电话。dàole yǐhòu, wǒ gěi nǐ dǎ diànhuà.
I will call you after I arrive.
2. Changed circumstances.
The particle‘了’occur the sentence to express the changed circumstances.
A1他是教授。 tā shì jiàoshòu.
He is a professor.
A2他是教授了。tā shì jiàoshòu le.
He is a professor now (after ten years working hard).
B1 天气很冷。tiānqì hěn lěng.
It is cold.
B2 天气很冷了。tiānqì hěn lěng le.
It is cold now (because winter to come).
3. The continuous aspect of an action or a state.
The aspect particle‘着’is placed directly after a verb to express the continuous aspect of an action or a state.
The door is open.
书在桌子上放着。 shū zài zhuōzi shang fàngzhe.
The book is on the table.
4. Past experience.
The aspect particle‘过’is placed after a verb to emphasize experience.
他去过中国。tā qùguo zhōngguó.
He has been to China.
我从来没见过她男朋友。wǒ cónglái méi jiànguo tā nán péngyǒu.
I have never seen her boyfriend.
5. An action that is going to take place in a short time.
The pattern ‘要(快) +Verb+了’is used to express an action that is going to take place in a short time.
她要结婚了。tā yào jiéhūn le.
She is going to get married.
明天我们要考试了。míngtiān wǒmen yào kǎoshì le.
We are going to take a exam tomorrow.
6. The progressive aspect of an action.
The words‘正在(正、在)’in placed in front of the verb to express the progressive aspect of an action.
你在做什么？nǐ zài zuò shénme?
What are you doing?
他正在做作业。tā zhèngzài zuò zuòyè.
He is doing his homework.