when和while
when和while都可表示“当……的时候”，引导时间状语从句。你知道两者在过去进行时中的用法有什么不同吗？
请仔细观察下面的例句，然后补全空格中的内容。
①I was walking to school when I saw a cat in a tree.
②When Sally arrived home, her mother was cleaning the house.
主句的动作是持续性的，而从句动作是短暂性的，此时用_______引导时间状语从句，主句用_____________时态，从句用一般过去时态。
③While I was sleeping in the bedroom, someone knocked at the door.
④David fell while he was riding his bike.
主句的动作是短暂性的，从句的动作是持续性的，此时用_________引导从句，主句用____________时态，从句用过去进行时态。
⑤While Andy was waiting for the bus, he was reading a newspaper.
⑥He was cleaning his car while I was cooking.
主句和从句的动作在过去的某时刻或某段时间同时发生，而且动作都是持续性的，此时用_________引导从句，而且主句和从句都用_____________
时态。
练习：用when 和while 填空
1. We were singing __________ the teacher came in.
2. _________ I was doing my homework , my mother was cooking.
3. What were you doing _________ the rainstorm came ?
4. ________ I was waiting for the bus at the bus station, it began to rain.
5. ________ he woke up, the smoke was rising.
