Question

什么时候要用名词性的物主代词：就是mine，yours，his，hers，its，ours，theirs。什么情况下可以用

Answer 1

He is one of my friends = He is one friend of mine.
Your book looks new, but mine looks old.
名词性物主代词在相当于形容词性物主代词+名词的时候就可以用。即：
名词性物主代词=形容词性物主代词+名词

追问

再问你一下，who is that。后面能不能说 ：that is hers

追答

你是说Whose is that?
答语应该是It is hers. that要变成it.

追问

为什么，that要变成it

追答

What's this? It's a book.
道理跟上面的问答句式一样。
因为指示代词有远指、中指和近指之分，对说话者和听话者而言that 和this可能不同。而it则指说话者和听话者都明确的它，故而有这样的变化。