is加現(xiàn)在分詞是什么時態(tài)

　　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.