英语中主格,所有格使用方法英语中什么时候应该用ther 什么时候用their 什么时候用them

they,他们,代名词,名词性质,主格
their,他们的,定语,名词性质的用theirs
them,他们,代名词,名词性质,宾格
代词还有：
I my mine me
例如：
I have a red book.
My book is red.
Mine is a red book.
The red book is for me.
其它代词：
he his his him
she her hers her
we our ours us
you your yours you
they their theirs them