The auxiliary word "了(le)" means that something has changed, and it is used to remind the listener of the new situation or new information. Here are several structures for this usage:
Used at the end of a sentence, "了(le)" means that something has happened. For example:
wǒ gēn tā duàn jué guān xì le.
I have broken up with her. (She and I are not friends any more)
tā qù dà shǐ guǎn lǐng qiān zhèng le.
He went to the embassy to get his visa. (He has gone)
tā bān zǒu le.
He has moved. (He has left this place)
noun or adjective+了
"了(le)" is also used at the end of a sentence to indicate that something has changed. Examples:
zì cóng tā piàn wǒ yǐ hòu, wǒ jiù bú xìn rèn tā le.
I have distrusted her ever since she cheated me. (I don't trust her now)
tā tí chū le pī píng, wǒ fǎn ér gèng xǐ huān tā le.
I like him all the better for his criticism. (I like him now)
dōu dà xué shēng le, nǐ kě yǐ dú lì shēng huó le.
You are an undergraduate now and you can live on your own. (You are no longer a primary or secondary school student)
verb+了+ measure word+了
"了(le)" emphasizes that an action has sustained for a long time. The action might continue or not, depending on the following sentence. Examples:
wǒ xué hàn yǔ xué le yī nián le, wǒ hái yào xué yī nián.
I have been learning Chinese for a year and I will take one more year. (I will continue learning)
yī wǎn shàng tā dōu xiě le wǔ piān wén zhāng le, gāi xiū xī le.
He has written five articles within one night and he should take a break. (He won't continue writing)
This structure can be used to indicate comparison and changes. Examples:
tā xiàn zài zhōng wén shuō dé liú lì duō le.
He speaks Chinese much more fluently now.
xiàn zài quán qiú de qì hòu nuǎn huo duō le.
The global climate becomes warmer now.
要/快/快要/就要 … 了
It means that something is about to happen. Examples:
yào xià yǔ le, kuài huí jiā ba.
It's going to rain. Go home now.
fēi jī mǎ shàng jiù yào qǐ fēi le, qǐng dà jiā jì hǎo ān quán dài.
The plane is about to take off. Please buckle up.
别/不要 … 了
It means to stop doing something. Examples:
bú yào shēng qì le, tā yòu bú shì gù yì de.
Don't be angry. He didn't do it on purpose.
"了(le)" also indicates that you feel happy when something happens unexpectedly. Examples:
nín néng lái cān jiā wǎn huì, zhēn shì tài hǎo le!
It's so great to have you in this party!
rú guǒ kě néng, jiè lái yī diǎn nà jiù zuì hǎo le.
If possible, it would be fine to borrow a little.