is加現(xiàn)在分詞是什么時(shí)態(tài)

　　句型分析：

　　1、一般疑問(wèn)句是將be動(dòng)詞移到主語(yǔ)前面，句末加問(wèn)號(hào)，讀升調(diào)。

　　be+主語(yǔ)+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(dòng)詞后加not。

　　主語(yǔ)+be+not+doing sth

　　I am not working.

　　He isn't running on the track.

　　The students aren't playing football.