不

不 (bù) is the all-purpose Mandarin negator, placed directly before a verb or adjective to deny it: 不是 'is not,' 不去 'not going,' 不好 'not good.' It works for present and future actions and for general states, but it cannot negate the verb 有 — for that you must use 没 (méi). Two things trip English speakers up: first, 不 is a stand-alone negator, not equivalent to English 'don't' or 'won't' — Chinese verbs do not change form, so 我不吃 covers 'I don't eat,' 'I won't eat,' and 'I'm not eating.'

Second, the tone of 不 shifts to bú when followed by a fourth-tone syllable (不是 bú shì, 不要 bú yào). 不 is also the standard one-word answer for 'no' to a yes-no question.