别: don't (negative imperative)
副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
别 + Verb (+ 了)
别 (bié) tells someone not to do something: it's the negative imperative, "don't!".
别 + verb: advice or prohibition (别担心 = don't worry).
别 + verb + 了: asks someone to stop an ongoing action (别说了 = stop talking, that's enough).
别 means the same as 不要, but is shorter and more colloquial.
Examples
别担心，你的病不严重。
Bié dānxīn, nǐ de bìng bù yánzhòng.Don't worry, your illness isn't serious.
别喝冰水。
Bié hē bīngshuǐ.Don't drink iced water.
别玩手机了，快睡觉吧！
Bié wán shǒujī le, kuài shuìjiào ba!Stop playing on your phone, go to sleep!
别忘了吃药。
Bié wàngle chī yào.Don't forget to take your medicine.
Common mistakes
别 doesn't negate facts: ✗ 我别去 → ✓ 我不去.
别 is addressed to someone (you / us): 我们别等了 (let's not wait any longer).