Past tense with particle ‘了‘ (1) – placed after a verb to denote the completion or realization of an action.
Verb + 了
Please compare the sentences in groups A and B:
Nǐ chī shénme?
What are you going to eat?
Nǐ chī le shénme?
What did you eat?
Nǐ qù nǎr?
Where are you going?
Nǐ qùle nǎr?
Where did you go?
Obviously, in group A, ‘吃烤鸭’ and ‘去超市’ are not completed action in the past, In group B, these two actions are definitely completed or realized.
Past tense with particle ‘了'(2) – placed at the end of a declarative sentence.
Statement + 了
① it emphasizes the completion or realization of some events or situations.
09. 你刚去哪儿了？ – Where have you been ? / Where did you go?
Nǐ gāng qù nǎr le?
10. 我去超市了。- I have been to the supermarket.
Wǒ qù chāoshì le.
11. 你买什么了？- What have you bought?
Nǐ mǎi shénme le?
12. 我买裙子了。- I have bought some skirts.
Wǒ mǎi qúnzi le
② it indicates a change in the situation or the occurrence of a new situation.
13. 我现在开始喝咖啡了。- Now I start drinking coffee.
Wǒ xiànzài kāishǐ hē kaāfēi le.
14. 刚刚还是晴天，现在就下雨了。- It was sunny but it is raining now.
Gāng gāng hái shì qíngtiān, xiànzài jiù xiàyǔ le.
15. 我上个月买的手表，这个月就被偷了。- The watch I bought last month, it was stolen this month.
Wǒ shànggè yuè mǎi de shǒubiǎo, zhè gè yuè jiù bèi tōu le.
16. 上午很热，下午就变冷了！- It was hot in the morning, it is getting cold in the afternoon!
Shàngwǔ hěn rè, xiàwǔ jiù biàn lěng le.
If you want to negate this kind of sentences – something that did not happen in the past – you have to use ‘有’ or ‘没有’ (without le).
没（有）+ Verb + Object
17. 我今天没（有）喝咖啡。 – I didn’t drink coffee today.
Wǒ jīntiān méi (yǒu) hē kāfēi.
18. 我昨天没（有）去超市。- I didn’t go to the supermarket yesterday.
Wǒ zuótiān méi (yǒu) qù chāoshì.
19. 我没（有）去游泳。- I did not go swimming.
Wǒ méi (yǒu) qù yóuyǒng.
20. 我还没（有）吃晚饭。 – I haven’t had dinner yet.
Wǒ hái méi (yǒu) chī wǎnfàn.
* ‘还’ can be used here to indicate that something ‘has not yet happened’.