不【bù】

不 is the everyday negator. It goes directly in front of whatever it negates – a verb, an adjective, or a modal verb.

不 or 没

不 negates habits, states, choices and plans: what someone does or doesn't do, and what they will or won't do. 没 negates an event that didn't happen.

• × 昨天我不去 → 昨天我没去 (for "I didn't go yesterday")

有 always refuses 不: its negative is 没有.

• × 我不有钱 → 我没有钱

With a modal verb

不 goes before the modal, not before the main verb. 我想不去 means "I'm thinking of not going"; to say you don't want to go, it's 我不想去.

The tone changes before a fourth tone

不 is bù on its own and before first, second and third tones. In front of a fourth tone it shifts to bú: 不是 is búshì, 不去 is búqù, 不对 is búduì. The character stays the same; only the pronunciation changes.

In questions and answers

Repeating the verb around 不 makes a question, and 不 + the verb makes the answer.