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当bǜ遇到第四声的音节,需要变成第二声
举例 bǘ yào bǘ shì


不 (bù) is 4th tone except when followed by another 4th tone, when it becomes second tone. E.g. (bù duì -----bú duì) 不对
bù yào-------- b ǘ yào 不要
bù shì--------bǘ shì. 不是


This is called "Tone sandhi" or 變調 or 变调.

See
http://en.wikipedia.org/wiki/Tone_sandhi
or
http://zh.wikipedia.org/wiki/%E8%AE%8A%E8%AA%BF

When transcribing Chinese to Hanyu Pinyin, these tone changes are normally NOT written.

So you WRITE or transcribe 不对 => bù duì, but you SPEAK: bú duì.
thank you !I was too careless
Reading aloud a text including 不 (bù) without any rehearsal is difficult for me. I sometimes make a mistake.
You need more practice
This is also our the difficulty in the teaching
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