If you want to talk in general about someone, and you do not want to say if the person is male or female, you often use
they,
them, or
their:
If anyone doesn’t like it, they can leave. 如果有人不喜欢,可以离开。
When a friend upsets you, do you tell them? 如果一个朋友惹你生气了,你会告诉他们吗?
Someone has left their coat behind. 有人忘了拿大衣。
People do this in order to avoid suggesting that the person can only be male, or to avoid using longer expressions such as 'he or she', 'him or her'.
This use is acceptable and very common in spoken English, and is becoming more acceptable in writing as well.
However, some people consider this use to be incorrect. You can sometimes avoid the problem by making the subject plural:
If people don’t like it, they can leave. 如果有人不喜欢,可以离开。
When friends upset you, do you tell them? 如果朋友惹你生气了,你会告诉他们吗?
In the past, people used to use 'he' to refer to men or women, but most people now avoid doing this.