1. 陈述部分的主语是指示代词this, that或these, those时，附加疑问句的主语分别用it和they。
This is an English book, isn’t it?
These are English books, aren’t they?
2. 陈述部分的主语是someone，somebody，anyone ，everyone，nobody ，no one 等复合不定式时，附加疑问句的主语在非正式的文体中通常用复数they。
Nobody phoned me while I was out, did they?
Somebody took my pen by mistake yesterday, didn’t they?
