them(他她它们)是they的宾格，一般在介词后

their(他它她们的)是they的形容词性物主代词，后面加名词

they(他它她) 是代词，作主语。

Theyhave maintained their optimism in the face of desolating subjugation.

面对遭征服的悲惨命运，他们保持了乐观的态度。

Let's them speak for themselves.

让它们自己来回答吧。

That's bolstered their reputationas safe.

这提振了它们安全性的声誉。