1. Describing the past: Add 了 le either to the end
of the sentence or directly after the action verb:
张老师  前天 去 中国 了。 
Zhāng lǎoshī qiántiān qù Zhōngguó le.
张老师 前天 去了 中国。 
Zhāng lǎoshī qiántiān qù le Zhōngguó.
Teacher Zhang went to China the day before yesterday.

2. zhāng lǎoshi yĭjing qù Zhōngguó le.
Indicating the near past: Use adverbs such as 已经yĭjing (already)，刚gāng (just) to indicate the near past:
张老师 已经 去 中国 了。
zhāng lǎoshi yĭjing qù Zhōngguó le.
Teacher Zhang has already gone to China.

3. qù nián, tā shì jiāohuàn xuéshēng.
Verbs such as 是 shì (to be), 想 xiǎng (would like; to think), 忙 máng (busy) normally do not take 了 le.
去年， 他 是 交换 学生。 
qù nián, tā shì jiāohuàn xuéshēng.
Last year, he was an exchange student.
他 昨天 很忙。 
tā zuótiān hěn máng.
He was very busy yesterday.

4. Negating action verbs- use 没有 méiyǒu
Affirmative
昨天 我 去 伦敦 了。
Zuótiān wŏ qù Lúndūn le.
I went to London yesterday.
Negative
昨天 我 没（有） 去 伦敦。
Zuótiān wŏ méi(yǒu) qù Lúndūn.
I didn’t go to London yesterday.

5. In both of the constructions above, 了 le must be kept in the question.
Alternative ways of asking and answering yes-no questions in the past Instead of using 吗ma, add 没有 méiyǒu to the end of a sentence: 昨天 你 打 太极拳 了 没有? Zuótiān nĭ dă tàijíquán le méiyǒu? Did you practise t’ai chi yesterday?
In both of the constructions above, 了 le must be kept in the question.

6. Using the familiar verb + negation word + verb construction
Note, however, that the negation word 没 méi (rather than 不 bù) is used in this context and了 le must not be used:
昨天你打没打太极拳? 
Zuótiān nĭ dă méi dă tàijíquán?
Did you practise tai chi yesterday?

7. Negating state verbs and adjectives – use不 bù
两年前， 他 不是 老师。 
Liǎng nián qián, tā bú shì lǎoshī.
lit. Two years ago, he not be teacher. (Two years ago, he wasn’t a teacher)
他 昨天 不忙 。 
Tā zuótiān bù máng.
lit. He yesterday not busy. (He wasn’t busy yesterday)