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

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
再问： 再问你一下，who is that。后面能不能说 ：that is hers
再答： 你是说Whose is that? 答语应该是It is hers. that要变成it.
再问： 为什么，that要变成it
再答： 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。