你说的对!
举例说明吧.
例如：
Which 【is the book 】you want?
Which 【are the books】 you want?
作主语,单复数取决于指代对象的单复数.
【Which one is 】yours?
【Which ones are】 theirs?
作定语,单复数由所修的名词或代词的单复数决定.
祝你开心如意!