句型分析：
1、一般疑问句是将be动词移到主语前面，句末加问号，读升调。
be+主语+doing sth
Are they putting up the scaffolding? Yes, they are. / No, they aren't.
Is he showing a foreign guest round the city? Yes, he is. / No, he isn't.
2、否定句式是在be动词后加not。
主语+be+not+doing sth
I am not working.
He isn't running on the track.
The students aren't playing football.