八(bā) - eight in Mandarin

八 (bā) is eight, and it is the number Chinese speakers actually want. It sounds close to 发 (fā), as in 发财 (fācái, to get rich), so phone numbers and number plates full of 八 sell at a premium, and the Beijing Olympics opened at 8pm on 8 August 2008. Nobody pretends this is anything but superstition, and nobody turns down an 八 either.

Grammatically it behaves like every other numeral: straight in front of a measure word, as in 八个人 (bā ge rén, eight people), 八本书 (bā běn shū, eight books) and 八点 (bā diǎn, eight o'clock). 八月 (bāyuè) is August, 八号 (bā hào) is the eighth of the month, and eighth in a sequence is 第八 (dì bā).

On the tone: the dictionary form is a flat first tone, and that is what you should say. Some speakers, especially in and around Beijing, lift 八 to a rising bá before a fourth-tone syllable, so 八个 can come out as bá ge, just as 七 (qī) sometimes becomes qí. This is an optional colloquial habit, not a rule like the compulsory changes on 一 (yī) and 不 (bù), and it is fading. Recognise it when you hear it; do not copy it on purpose.