表示两个动作并列,同时发生
例：I'm doing my homework while he is watching TV.
表示正在做某事时突然发生了什么
例：I was cooking when I heard the phone ring.
表示动作先后发生
例：I walked into the room and turned on the light.
在副合句中主句和从句都有谓语时
例：I told her a thing she didn't know.
这是我总结的,希望能帮到你