Table of Contents
These two common words change tone according to the syllable that follows them.
In Part 1 of this series, we saw how neighbouring third tones interact. Two other characters change so often that their alternative pronunciations should become automatic: 不 (bù) and 一 (yī). The rules are short, predictable and useful from your first conversation.
When 不 (bù) changes to bú
The following syllable determines which pitch path the word takes.
不 (bù) normally has a falling fourth tone. Before another fourth-tone syllable, it changes to the rising second tone bú:
不是 (bú shì): is not; am not
不要 (bú yào): do not want
不用 (bú yòng): no need
不错 (bú cuò): not bad
Before a first, second or third tone, it remains bù: 不吃 (bù chī) means do not eat; 不忙 (bù máng) means not busy; and 不好 (bù hǎo) means not good.
The quick rule is therefore: fourth tone before fourth tone becomes second tone. Learn 不是 (bú shì) as one spoken chunk instead of stopping to calculate it each time. For the grammar behind common negatives, see our guide to 不 (bù) versus 没有 (méiyǒu).
When 一 (yī) becomes yí or yì
一 (yī) keeps its first tone when said alone, counted, or read as part of a number: 一、二、三 (yī, èr, sān) means one, two, three.
Before a fourth tone, it becomes the rising yí:
一个 (yí ge): one; one item
一次 (yí cì): once
一件 (yí jiàn): one item of clothing
Before a first, second or third tone, it becomes the falling yì:
一天 (yì tiān): one day
一年 (yì nián): one year
一本书 (yì běn shū): one book
A useful contrast is 一个 (yí ge) versus 一本书 (yì běn shū). A common misunderstanding is that the printed pinyin must always show the spoken change. Conventions vary for these words, so focus on the audio and recognise both the dictionary form and its contextual pronunciation.
Both rules reduce awkward sequences: the voice alternates direction instead of producing two consecutive falling fourth tones.
Practise without calculating
Make two short drills. First alternate 不是 (bú shì), 不要 (bú yào) and 不好 (bù hǎo). Then repeat 一天 (yì tiān), 一年 (yì nián), 一本书 (yì běn shū) and 一个 (yí ge). Record the list, listen back and correct only one pattern at a time.
When ordering in China, say the full measure-word phrase smoothly: 一个咖啡 (yí ge kāfēi) means one coffee; more naturally, say 一杯咖啡 (yì bēi kāfēi) for one cup of coffee. Do not pause after 一 (yī); the following syllable tells your voice where to move.
Useful Chinese Words and Phrases
- 不是is not; am not
- 不用no need
- 一个one; one item
- 一本书one book
- 一杯咖啡one cup of coffee
A Short Everyday Conversation
Customer: 你好，我要一杯咖啡。
Nǐ hǎo, wǒ yào yì bēi kāfēi.
Hello, I would like one cup of coffee.
Server: 要糖吗？
Yào táng ma?
Would you like sugar?
Customer: 不要糖，谢谢。
Bú yào táng, xièxie.
No sugar, thank you.
Final Takeaway
Before a fourth tone, 不 (bù) becomes bú, while 一 (yī) becomes yí. Before first, second or third tones, 一 (yī) becomes yì. Drill whole phrases until the next syllable triggers the change automatically.