Chinese Basic 1
Origin and Languages - Language Notes

Negation - 不

To negate a statement you can use the adverb 不 bù before the verb.
For example:
我不是学生。 Wǒ bù shì xuésheng. I am not a student.
他不说普通话。 Tā bù shuō Pǔtōnghuà. He does not speak standard Chinese.

Note that 不 bù is pronounced bú in front of a syllable carrying a fourth tone. So 不是 is pronounced bú shì. This change in tone is normally not made explicit in Pinyin, so you will still see bù shì.