Are 是b动词，they是主语。
Are they...句式表示疑问，意思为：他们是……吗？
Are they...的句式中b动词提至主语they前表示疑问。Are they 与They are的用法区别举例如下：
1、What are they? They are cats. 他们是什么？他们是猫。
2、What are they? They are teachers. 他们是干什么的？他们是老师。
