The word bù (不, meaning "no" or "not") is one of the most common words in Mandarin. It normally carries a 4th Tone, a sharp, falling pitch. But in one specific situation, its tone changes. This is another example of tone sandhi, and it is a rule you will use every day.
The Rule
When bù appears directly before another 4th Tone syllable, it changes from the 4th Tone to the 2nd Tone (rising). In all other cases (before a 1st, 2nd, or 3rd Tone) it stays as the 4th Tone.
Why This Happens
Two falling tones in a row create an awkward, heavy rhythm. The shift from a 4th Tone to a 2nd Tone before another 4th Tone syllable keeps the speech flowing smoothly. It is a natural adjustment that every native speaker makes without thinking.
Examples: Before a 4th Tone (bù Changes)
- bù shì (Is not) → spoken as bú shì, because 'shì' is 4th Tone
- bù duì (Not correct) → spoken as bú duì, because 'duì' is 4th Tone
- bù yào (Do not want) → spoken as bú yào, because 'yào' is 4th Tone
Examples: Before Other Tones (bù Stays)
- bù chī (Not eat) → stays as bù chī, 'chī' is 1st Tone
- bù xíng (Not okay) → stays as bù xíng, 'xíng' is 2nd Tone
- bù hǎo (Not good) → stays as bù hǎo, 'hǎo' is 3rd Tone
How to Remember It
The pattern is straightforward: bù only changes before the 4th Tone. A simple way to remember: two falling tones clash, so the first one rises to avoid the collision.
Just like the 3rd Tone sandhi rule, the written Pinyin often shows the original tone (bù), but some textbooks and dictionaries helpfully write the changed tone (bú) when it appears before a 4th Tone. Either way, the spoken rule is always the same.
When bù Goes Neutral
Besides rising before a fourth tone, bù has a third behavior many courses skip: it can lose its tone entirely. Tucked inside a verb, in the common A-not-A question pattern and in potential complements, bù becomes light and toneless. You hear it in hǎo bu hǎo (好不好, is it okay?), kàn bu kàn (看不看, watch or not?), and chī bu bǎo (吃不饱, cannot eat enough). In these, do not stress the bù or give it a pitch; let it slip by quickly and unaccented.
Practice
Read these phrases aloud and apply the rule:
- bù cuò (Not bad): does 'cuò' carry a 4th Tone? Yes. So say bú cuò.
- bù lái (Not come): does 'lái' carry a 4th Tone? No, it is 2nd Tone. So say bù lái.
Check the tone of the syllable after 'bù' first. If it falls, 'bù' rises. That is the entire rule.
