句型分析：
1、一般疑問句是將be動詞移到主語前面，句末加問號，讀升調。
be+主語+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動詞后加not。
主語+be+not+doing sth
I am not working.
He isn't running on the track.
The students aren't playing football.