when的用法四种句型：be doing sth…when；be about to do sth…when；hardly…when；had just done…when。
1、be doing sth…when
例：he was still smiling when the door opened and his wife came in.
他正笑着的时候门突然开了，他妻子走了进来。
2、be about to do sth…when…
例：we were about to start when it began to rain.
我们刚要出发，天就开始下雨了。
3、had not done sth…when…/hardly…when…
例：he had not fallen asleep when the telephone rang.
他刚要入睡电话就响了。
4、had just done…when…
例：i had just gone to bed after a very hard day when the phone rang.
在劳累了一天之后我刚刚就寝，电话铃就响了。