used in positive statements when we do not know exactly which person we are referring to我们无法确切知道我们指的是哪个人时候，用于肯定/积极陈述2. can also be used to make requests可以用来提出要求/请求3. used in questions and negative statements用于问题和消极/否定陈述中4. used to mean'not anyone/anybody用于意思是任何人....
