You use their to indicate that something belongs or relates to the group of people, animals, or things that you are talking about.
双语例句
例：
Janis and Kurt have announced their engagement.
贾尼斯和库尔特已宣布订婚。
2.?（用于代替“他的”或 “她的”，不指明是男是女。有些人认为这种用法不正确）
You use their instead of 'his or her' to indicate that something belongs or relates to a person without saying whether that person is a man or a woman. Some people think this use is incorrect.
双语例句
例：
Each student determines their own pace in the yoga class.