别 is the negative imperative — use it to tell someone NOT to do something. It always goes before the verb: 别 + verb. Compare with 不 (general negation) and 没 (negating past actions).
mistakes
Don't confuse 别 (bié, "don't") with 没 (méi). 别走 means "don't go" (a command about the future), while 没走 means "didn't go" (a statement about the past).
