不 (bù) is the most common negation word in Mandarin, meaning "not" or "no." It goes directly before a verb, adjective, or another adverb to negate it: 我不去 (wǒ bù qù) "I'm not going," 他不高 (tā bù gāo) "He's not tall."

Tone sandhi: 不 is normally fourth tone (bù), but when it comes directly before another fourth-tone syllable, it changes to second tone (bú) in speech, though the pinyin is often still written as bù. Example: 不对 is pronounced bú duì, not bù duì.

不 vs 没 (méi) is a key distinction for learners: 不 negates general states, habits, opinions, and future actions (不喜欢 "don't like," 不去 "won't go"), while 没 negates completed actions or possession in the past (没去 "didn't go," 没有钱 "don't have money"). You generally can't say 不有 — 有 is negated only with 没.

As a character, 不 is a simple pictograph, traditionally showing a bird flying upward, though the exact original meaning is debated — some scholars connect it to a flower calyx instead. It's one of the first characters most learners memorize, and it appears in countless set phrases like 不客气 (bú kèqi, "you're welcome") and 不错 (bú cuò, "not bad").