不【bù】

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

With a modal, 不 attaches to the modal, not to the main verb. wǒ我xiǎng想bù不qù去 is not the way to refuse; say 我不想去.

What 不 negates

不 covers what is not true in general, what you choose not to do, and what you are not going to do. It says nothing about the past.

没 for what did not happen

An event that failed to take place takes 没, not 不. zuó tiān昨天wǒ我bù不qù去 is wrong for "I didn't go yesterday" – it has to be 昨天我没去.

有 is the one verb that always refuses 不: the negative of 有 is 没有 at every level of the language.

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 never changes, only the pronunciation.

不 inside a question and an answer

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