初二下Unit 3What were you doing when the UFO arrived?
问题描述：
初二下Unit 3What were you doing when the UFO arrived?
Listen!Some bird (were singing) in the tree.
为什么填were singing,而不是are singing?
答
这个时态是过去进行时态.因为当你看到一些鸟唱歌的时候.一些鸟唱歌是在你看到之前就开始的,只是持续到现在的.希望对你有所帮助.我英语系的,以后你不清楚可以问我.