A lot of Chinese learners believe that 了 *means* the past tense in Chinese.
But that’s not quite true…
了 is not a character with a tangible meaning that can be translated
To say it refers to the past tense is too simplistic, it has a number of different usages.
了 often indicates that a situation has changed
In this case, its meaning could be interpreted as: ‘and that’s the situation now’.
Sometimes it can be used similarly to ‘ya’ in Spanish, e.g. Ya esta – that’s enough (够了，可以了）
Some examples：
下雨了 xià yǔ le – It’s raining (and that’s the situation now)
我老了 wǒ lǎo le – I’m old (now) (I wasn’t before)
好了hǎo le – It’s ok (now) (it wasn’t before)
我换工作了 wǒ huàn gōngzuò le – I’ve changed jobs
Sometimes 了 marks an action as complete, but that action can be in the past, present or future.
我已经说了 wǒ yǐjīng shuōle – I’ve already said (it).
我买了三本书 wǒ mǎile sān běn shū– I bought three books.
我学了三年中文 wǒ xuéle sān nián zhōngwén I’ve been learning Chinese for 3 years.
我到了市中心 wǒ dàole shì zhōngxīn – I’ve arrived in the city centre (I’m in the city centre).
我毕业了以后，想当老师 wǒ bìyèle yǐhòu, xiǎng dāng lǎoshī – After I graduate, I want to be a teacher.
Many actions that happen in the past are in fact ‘completed’, that’s why you often see 了 when referring to past events
我昨天去了上海 wǒ zuótiān qùle shànghǎi I went to Shanghai yesterday (this is a completed event).
我买了一些东西 wǒ mǎile yīxiē dōngxī
I bought some things (completed event)
我已经吃了 wǒ yǐjīng chī le
I have already eaten (completed event)
你昨天去哪里了？nǐ zuótiān qù nǎlǐ le Where did you go yesterday?
In 太 + adjective + 了
This usage is a ‘set expression’, it is not related to the previous usages of 了.
太好了 tài hǎo le – That’s great.
太棒了 tài bàng le– That’s awesome.
这座城市太大了 zhè zuò chéngshì tài dàle This city is really big.
In adjective + 极了 – ‘extremely’ something
This usage is also used in ‘set expressions’.
棒极了 bàng jí le – That’s extremely good.
你的英文好极了 nǐ de yīngwén hǎo jíle
Your English is extremely good
In adjective + 死了 – ‘so’ something
This usage is also used in ‘set expressions’.
我冷死了 wǒ lěng sǐ le – I’m so cold.
我想死你了 wǒ xiǎng sǐ nǐ le
I miss you so much.
烦死了 fán sǐ le – It’s so annoying.
可 + adjective +了
This usage is also used in ‘set expressions’.
他对我可好了 tā duì wǒ kě hǎo le
He’s pretty good to me.
这个房子可贵了 zhège fáng zǐ kě guì le
This house is pretty expensive.
Note that 了 can sometimes be liǎo, which is used in completely different phrases.
This usage is also used in ‘set expressions’.
我等不了你 wǒ děng bù liǎo nǐ
I can’t wait for you
(I wait not finish you)
我做得了wǒ zuò déliǎo – I can do it
I do and the result is that I will complete it.
Once again:
了 can indicate that a situation has changed and ‘this is the situation now’. Eg. 下雪了 xiàxuě le- it’s snowing (it wasn’t before, but that’s the situation now).
了 can be used to mark a situation as ‘completed’.
Eg. 我已经去了 wǒ yǐjīng qù le
I’ve already been.
了 can be used in set constructions with:
太…了，可…了 or …死了 Tài…le, kě…le or… sǐle
Eg. 太好了，可好了，热死了
tài hǎo le, kě hǎo le, rè sǐ le