is加現在分詞是什麼時態

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動詞後加not。

主語+be+not+doing sth

I am not working.

He isn't running on the track.

The students aren't playing football.