The changing pronunciation of 不

[trying]

According to a book,

不 bu4 is pronounced in the 4th tone. But when it precedes another 4th tone word, it pronounced in the 2nd tone.

Ex. 不対(bu2 dui4), 不(bu2 shi4).

But I'm a little confused about 不要。　　I heard Chinese using tone 4 (bu4 yao4) when they use 不要 in the beginning of the sentence, but they use tone 2 when it's in the middle of a sentence, like 我不要再想下去　　（bu2 yao4).

So, can anyone kindly tell me what tone does the 不 take in 不要?

[HashiriKata]

I'd say it should always be /bu2 yao4/, even at the beginning of the sentence, but remember that our articulation of the first sound (and the last sound) of a sentence may be slightly different from that when the sound is in the middle of the sentence, and this may give you the impression that it's not in 2nd tone. Another thing is in a different sense, 要 is a 1st tone character, so when combined with 不, you'd have /bu4yao1/.

[againstwind]

HashiriKata is right. As a fixed phrase, 不要 should always be /bu2 yao4/.

And 要 is a polyphone, yao4 and yao1.

e.g. 不要求 /bu4 yao1 qiu2/

Considering there are so many people in China who speak Chinese in their own accents, it's not a surprise to hear someone make nonstandard tone.

[trying]

Thank you everyone for your replies.

[heifeng]

don't forget, 'bu' also has a qingsheng in some instances...

[trying]

Hi Heifeng,

Can you kindly give me a few examples? Thank you!!!

[heifeng]

Usually when it is wedged between stufff like this, it's pronounced with a 轻声

了不起

吃不了

去不去

要不要

Overall, here are some practice exercises for you

不信鬼神

不相上下

看不清

不算高明

不痛不痒

不听劝告

想不通

不见不散

不怕困难

不大可能

来不来

不干不净

不懂外语

不太合适

[trying]

Thank you Heifeng.

That's a lot of practice for me. Thanks

BTW, do anyone know where I can find info about counters for things.

For example, counting horse would use 一匹、counting snake is 一篠　（？）。

So, what about counting cats, dogs, elephant and so forth?

Thank you!!!!!!!!!!!!!!!

[raincoat]

Can try this Chinese - English Dictionary of Measure Words http://www.chinesemall.com/chindicofmea.html

[trying]

Thank you. I hope they have it in Hong Kong. I'm visiting HK in summer.