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