句中如果含有be动词,那么就将be动词作为助动词使用,帮助否定、疑问.若句中无助动词,如can等情态动词、be动词等时,就应该借助助动词do/does/did.
比较：1.He is a student.一般疑问句：Is he a student?否定句：He isn't a student.
2.She lives in China.一般疑问句：Does she live in China?否定句：She doesn't live in China.