1. 他们的，她们的，它们的(东西)
    You use theirs to indicate that something belongs or relates to the group of people, animals, or things that you are talking about.

    e.g. There was a big group of a dozen people at the table next to theirs...
           他们旁边的桌上坐了一大群人，有十几个。
    e.g. It would cost about £3,000 to install a new heating system in a flat such as theirs...
           在他们那样的一套公寓房里安装一套新的供暖系统可能要花费 3,000 英镑左右。

2. (用以代替his or hers，而不指明是男是女;有人认为这种用法不对)
    You use theirs instead of 'his or hers' 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.

theirs在线翻译

    e.g. He would leave the trailer unlocked. If there was something inside someone wanted, it would be theirs for the taking...
           他不会给那个活动房屋上锁。里面要是有什么别人想要的东西，他们可以尽管拿。
    e.g. This child is nobody's child until someone makes her theirs officially.
           在有人办理正式手续认领以前，这个孩子不属于任何人。