2. 过去进行时的结构及句型变化

　　过去进行时由"was / were + 现在分词"构成。如：

　　I was doing my lessons then. 那时，我在做功课。

　　We were cleaning the house. 我们在打扫房子。

　　过去将来时态的肯定句、否定句和疑问句形式：

　　肯定句例句：I was walking down the street when a UFO landed.

　　否定句例句：I wasn't walking down the street when a UFO landed.

　　一般疑问句例句：Were you walking down the street when a UFO landed?

　　特殊疑问句例句：What were you doing when a UFO landed?