Tone Changes - 不 (bù) and 一 (yī)

This grammar works by looking at the word that follows. You must check the tone of the next syllable. The next syllable dictates if 不 or 一 stays the same. Or if they transform into a different tone. It is all about the physical effort of speaking. Pronouncing two 4th tones in a row is tiring. It feels like stomping your feet twice very hard. To fix this, Chinese softens the first stomp. The same logic applies to the number 一. It is a high, flat 1st tone. Moving from a high flat tone to a low tone is a jump. The tone changes act as a bridge for your voice. They make the transition between words feel like a slide. Your tongue will definitely thank you for this. It is like shifting gears in a car. You do not just stay in one gear all day. You adjust to the road ahead. In this case, the road is the next word in your sentence.

1. 1For the word 不 (bù):
2. 2If the next word is 1st, 2nd, or 3rd tone, keep 不 as 4th tone (bù).
3. 3If the next word is 4th tone, change 不 to 2nd tone (bú).
4. 4Example: 不是 becomes bú shì instead of bù shì.
5. 5For the word 一 (yī):
6. 6If 一 is alone or at the end, keep it 1st tone (yī).
7. 7If the next word is 4th tone, change 一 to 2nd tone (yí).
8. 8Example: 一个 becomes yí gè instead of yī gè.
9. 9If the next word is 1st, 2nd, or 3rd tone, change 一 to 4th tone (yì).
10. 10Example: 一天 becomes yì tiān instead of yī tiān.

Use these rules every time you speak. They are not optional for natural speech. Use them when you are ordering food at a restaurant. For example, when you say 一个 (yí gè) for "one of something." Use them when you are declining an invitation. You will say 不去 (bú qù) for "not going." Use them when you are describing quantities. Saying 一共 (yí gòng) for "altogether" requires a tone change. Even in a job interview, these rules apply. They show you have a high level of language awareness. It makes you sound like you have lived in China. Use them when asking for directions. If you say 一直走 (yì zhí zǒu) for "go straight," use the 4th tone for 一. It helps the listener understand you without extra effort. It is like wearing a matching outfit. It just looks and sounds right to everyone around you.

Do not change the tone of 一 when counting. If you are saying "one, two, three," keep it as yī. Do not change it in phone numbers or room numbers. In those cases, it stays as the original 1st tone. Do not change it for ordinal numbers. For example, 第一 (dì yī) for "first" always stays 1st tone. For the word 不, do not change it if it is alone. If someone asks "No?" and you say 不 (bù), keep it 4th. Also, do not change it if it is the middle syllable. In potential complements like 对不起 (duì bù qǐ), it is neutral. It is like a holiday for your tones. They get to rest and stay in their original form. Knowing when to stop is just as important as knowing when to change. It prevents you from over-correcting your speech.

Many people forget to change 不 before 是 (shì). They say bù shì and it sounds very heavy. Another mistake is keeping 一 as 1st tone everywhere. This makes your sentences sound choppy and disconnected. Some learners try to change the tone in written Pinyin. Usually, Pinyin in books shows the original tone. You are expected to know the change rule yourself. Do not get confused by the visual marks on the page. Trust your ears and the rules you learned. Another common error is changing the tone during counting. If you say yì, èr, sān, it sounds like you are starting a sentence. It does not sound like a list. Avoid over-thinking every single syllable while talking. If you miss one, do not panic. The grammar police will not arrest you. Just try to catch the next one.

This is different from the 3rd tone sandhi. In 3rd tone sandhi, two 3rd tones change to 2nd and 3rd. That rule applies to thousands of different words. The rules for 不 and 一 are special. They only apply to these two specific characters. Think of them as the "VIPs" of tone changes. They have their own private club rules. 3rd tone sandhi is like a general rule for the public. These rules are more specific and frequent. Another contrast is with neutral tones. Neutral tones lose their shape entirely. 不 and 一 do not lose their shape. They just swap one clear tone for another clear tone. It is a transformation, not a disappearance. Understanding this distinction helps you organize your mental grammar map.