The word “bù” (不) means 'no' or 'not', and is used to create a negative form of a verb in a sentence. But because of how the tones work in Chinese, “bù” changes its pronunciation in some cases. It's really not that difficult to deal with, but because doing it right seems to take practice, here's how it works.
When “bù” comes into contact with a character pronounced using the fourth tone, it changes its tone to the second tone. This occurs whether the following character is a verb or adjective.
• 不错 (bù + cuò = bú cuò)
• 不太 (bù + tài = bú tài)
• 不去 (bù + qù = bú qù)
• 不对 (bù + duì = bú duì)
For other characters, “bù” retains its fourth tone pronunciation. Do not change the pronunciation unless the next character uses the second tone. In the two examples below, both common phrases that you might use several times a day, the tone of the following character stays the same.
• 不忙 (bù + máng = bù máng)
• 不好 (bù + hǎo = bù hǎo )
Also, this rule only applies to “bù” (不) meaning 'not' or 'not, and has no bearing on other characters that happen to be pronounced with "bu" in the fourth tone. There are many words like this in Chinese, and don't change the tone on the first character in any of them.
• 部队 (bùduì; it does not change)
The reason for this phonetic change for “bù” (不) is that doing so avoids repeating two fourth tones, which is perceived as unappealing. It is, in other words, a matter of euphony to the Chinese, and must be respected as such.
None of this would cause much trouble, except that textbooks vary in how they deal with this change in the tones of “bù” (不). Some textbooks use tone markers in the Pinyin with the phonetic change noted, and others do not, though the phonetic change does occur. The accompanying audio (if available) will of course reflect the change in the tone of “bù” even if the printed text does not.
A little practice goes a long way to mastering the pronunciation of “bù” (不). It will soon enough become second-nature to make this change, just as in spoken English it is second nature to adjust "a" and "an" depending on the word that follows. So practice, pay attention, and in no time this pronunciation pattern will become second nature.