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Whenever 不 is before another fourth tone character, it becomes a second tone. 
Take for example, 不错 bùcuò, which means not bad. Since 错 is fourth tone, we read it as búcuò.

 Second tone sandhi rule

Ever had someone else ruining a date between you and your crush? Or just a friend that's ruining the mood between you and another? A third wheel? In Chinese we call that  
 
电灯泡
diàndēng pào

Example:

你今晚和我们一起去看电影吗？

nǐ jīn wǎn hé wǒmen yīqǐ qù kàn diànyǐng ma?

Are you going to the movie with us tonight?

不了，我不想当电灯泡哈哈。

Bùle, wǒ bùxiǎng dāng diàndēngpào hāhā。

Nah, I don't want to be a "light bulb" haha.