(a) Indicating a change of situation
Used to urge or order the listener(s) to stop doing something, 了 indicates a change of situation.
他已经向你道歉了；别哭了。
Tā yǐ jīng xiàng nǐ dào qiàn le ；bié kū le 。
He already apologized to you; don’t cry anymore.
(The listener is crying.)
我要告诉你一个坏消息；你听了以后，别哭。
Wǒ yào gào sù nǐ yī gè huài xiāo xī ；nǐ tīng le yǐ hòu ，bié kū 。
I am going to tell you some bad news. Don’t cry after you hear it.
(The listener is not crying.)
(b) Indicating the result of an action
了 can also indicate the result of an action. In this case, the imperative sentence can be a positive one as well.
别忘了把这封信交给他。
Don’t forget to hand this letter to him.
(了 does not indicate a change of situation; it is the result of 忘.)
Compare:
忘了他吧！像他这种人，不值得你爱。
Wàng le tā ba ！xiàng tā zhè zhǒng rén ，bú zhí dé nǐ ài 。
Forget him! Someone like him does not deserve your love.
儿子：这么多空瓶子，怎么办?
ér zǐ ：zhè me duō kōng píng zǐ ，zěn me bàn ?
Son: So many empty bottles. What are we going to do with them?
妈妈：丢了，别留那种没有用的东西。
Mā mā ：diū le ，bié liú nà zhǒng méi yǒu yòng de dōng xī 。
Mother: Throw them away. Don’t keep those kinds of useless things.
Compare:
这样东西很重要，拿好，别丢了。
Zhè yàng dōng xī hěn zhòng yào ，ná hǎo ，bié diū le 。
This is something very important; hold on to it; don’t lose it.
[1] Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.
