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Bù (不, not) is normally fourth tone. What happens to it right before another fourth tone, as in bù + shì?

It changes to second tone: bú shì.

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  1. It changes to second tone ✔
  2. It becomes silent
  3. It changes to third tone

Why: Bù rises to bú before a fourth tone, so 不是 is said bú shì and 不客气 is bú kèqi. Before first, second and third tones it stays bù.

Huh, didn't know that: Yī (one) also changes in many phrases: it is yí before a fourth tone and yì before a first, second or third tone, but stays yī when isolated.

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Even the word 'no' in Chinese knows how to compromise. Before a fourth tone, it goes up.
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References

Answer: It changes to second tone. Bù rises to bú before a fourth tone, so 不是 is said bú shì and 不客气 is bú kèqi. Before first, second and third tones it stays bù.

  1. Trinity University, Tone Change Rules in Mandarin Chinese faculty.trinity.edu/sfield/chin1501/ToneChange.html
    However, it is pronounced in the second tone when it precedes a fourth tone.
    Checked 2026-10-12.
  2. Trinity University, Tone Change Rules in Mandarin Chinese faculty.trinity.edu/sfield/chin1501/ToneChange.html
    YI (meaning "one") is pronounced in the first tone when it stands alone.
    Checked 2026-10-12.
  3. Trinity University, Tone Change Rules in Mandarin Chinese faculty.trinity.edu/sfield/chin1501/ToneChange.html
    It is pronounced in the fourth tone when it precedes a first, second, or third tone.
    Checked 2026-10-12.

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