必须【bìxū】

必须 goes before the verb and says there is no choice: "must", "have to". Rules, instructions and firm requirements use it.

It also states what a goal takes.

The negative is 不用 or 不必

必须 doesn't take 不. "Don't have to" is 不用 in speech and 不必 in writing.

• × 你不必须来 → 你不用来

To say "mustn't", use 不能 or 别.

• × 你必须不说 → 你不能说

必须, 得 or 应该

应该 is advice: you should. 必须 is a requirement: you must. 得děi says the same as 必须 but belongs to speech.