2.1 Tone changes
When two 3rd-tone syllables are together, the first syllable is usually changed to a 2nd tone. In other words, 你好 nǐ hăo is pronounced ní hăo, and 很好hěn hǎo would be pronounced hén hǎo, despite how they are marked in pinyin.
Now listen to how two third tones are first pronounced in isolation and then together with the tone change:
Transcript
nǐ hăo
ní hăo
When 不 bù (no, not) is followed by another forth tone, it changes to a second tone. For example, 不谢 bù xiè becomes 不谢 bú xiè.
In this course, syllables are usually marked in pinyin mostly as if they were in isolation: i.e. they do not reflect tone change. The only exception is the 4th-tone syllable 不 bù, whose tone change is reflected in pinyin, where 不 bù is marked with a 2nd tone, as in the expression 不谢 bú xiè.
Now listen to how the expression for ‘not at all, you are welcome’ is first pronounced without the tone change and then with the tone change.
Transcript
bù xiè
bú xiè
Here are some external links to help with pronunciation and tones:
Audio pinyin table: http://english.cri.cn/ chinese2007/ chinese/ english/ lesson01/ pinyin.html [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]
Pitch Perfect Pinyin: http://www.laits.utexas.edu/ ppp/