used to refer to a thing or things belonging to or associated with two or more people or things previously mentioned 他们的(所有物),属于他们的(东西);它们的 they think everything is theirs. 他们认为每件东西都是他们的。 a favourite game of theirs. 他们最喜爱的一种游戏。
语源
Middle English: from their + -'s
用法
There is no need for an apostrophe: the spelling should be theirs not their's